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Conservation tillage and sustainable intensification of agriculture: regional vs. global benefit analysis
Agriculture, Ecosystems and Environment 216 (2016) 155–165
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Conservation tillage and sustainable intensification of agriculture: regional vs. global benefit analysis Nikolaus J. Kuhna , Yaxian Hua,* , Lena Bloemertza , Jin Heb , Hongwen Lib , Philip Greenwooda a b
Physical Geography and Environmental Change Research Group, University of Basel, Klingelbergstrasse 27, Basel 4056, Switzerland Conservation Tillage Research Center, China Agricultural University, Qinghua East Road 17, Beijing 100083, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 20 June 2015 Received in revised form 30 September 2015 Accepted 1 October 2015 Available online xxx
Climate change is expected to affect both the amount of global crop production, and annual variability in food supply. Agriculture is a major source of greenhouse gas emissions, but also considered to mitigate climate change. Conservation tillage, as a climate-smart agricultural practice, is repeatedly reported to mitigate net greenhouse gas emissions by increasing soil organic carbon (SOC). However, with reduced tillage, less litter is moved from the surface deeper into the soil profile, so SOC increase is very likely constrained to topsoil layers. Further adaptation benefits, such as increasing crop yield and resilience to famine, have recently been questioned after averaging yields from field studies. However, such global averaging masks the geographic extent individual studies apply to. This paper attempts a holistic regional analysis on the benefits of conservation tillage, in particular its fundamental principle no-tillage (NT), on the Chinese Loess Plateau. Based on a review of almost 20 years of conservation tillage plot experiments, the potential of NT to increase SOC stocks and to adapt to lower but more variable rainfall in the future has been assessed. The results show that the difference of total SOC stocks between NT and CT decreased with soil depth, confirming that the SOC benefits of NT are concentrated to the immediate topsoil still subject to direct seeding. The topsoil achieved maximum SOC stocks after about 10 years of NT. Crop yields from NT increased by up to 20% for years with average and below average precipitation, demonstrating the advantages of NT in stabilizing crop yields in dry years. However, the results in previous reports are not weighted by the actual spatial extent of drylands and humid regions after counting individual plot studies. As a consequence of such global and unweighted averaging, the benefits from NT to increase SOC stocks are likely to misrepresent the actual impact. Therefore, given the size of the Loess Plateau and its relevance for food security in China, our analysis illustrates the need to assess the benefits of a tillage and residue management system for each combination of eco-region and farming practice, weighted by their area and the affected population, rather than just using a global average for policy development on sustainable productivity. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Conservation tillage Carbon sequestration Food security Global averaging Spatial weighting
1. Introduction Climate change is expected to affect global food security by changing both spatial and temporal patterns of yields (Wheeler and von Braun, 2013). Agriculture also causes emissions of 500 Tg C to the atmosphere per year (Lal, 2004a). In particular, the use of nitrogen fertilizers contributes 187–224 Tg CH4 and 1.7–4.8 Tg N2O per year (Ipcc, 2013). Meanwhile, agriculture could also mitigate climate change (Vermeulen et al., 2012), most notably by enhancing soil organic carbon (SOC) on degraded agricultural
* Corresponding author. E-mail address: [email protected] (Y. Hu). http://dx.doi.org/10.1016/j.agee.2015.10.001 0167-8809/ ã 2015 Elsevier B.V. All rights reserved.
lands. The contribution is equivalent to 50–60% of the 42–78 Pg C of soil carbon released from human-induced land use change over 25–50 years (Ipcc, 2013). In particular, conservation tillage, as a manner of climate-smart agriculture practice, is repeatedly reported to mitigate climate change by reducing net greenhouse gas emissions (Lal, 2004b; X. Wang et al., 2008; He et al., 2010b; Li et al., 2012). Conservation tillage may also contribute to climate change adaptation by increasing or at least limit the decline of crop yields in dry years (FAO, 2011), and thus help to strengthen resilience to famine by ensuring regional access to affordable food in dry years/dry regions (Cavatassi et al., 2011). Conservation tillage also has co-benefits that include increased resistance to soil erosion (Pimentel et al., 1995; Leys et al., 2007; Knapen et al., 2008), improving water use efficiency (Bai et al., 2009; He et al.,
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2010a; Li et al., 2011), and reducing labor and energy inputs (Tullberg et al., 2007). A fundamental principle of conservation tillage is no-tillage (NT), which minimizes mechanical disturbance of soil, limiting macro-aggregate destruction, and thereby reducing exposure of soil organic matter to mineralization (Rhoton et al., 2002; Six et al., 2004; Liu et al., 2014). While conservation tillage is in general considered to help increase crop yields and sequestrate atmospheric SOC (as discussed above), some recent papers argued that the potential contribution of NT to climate change mitigation and food security is more limited than initially thought. For instance, Powlson et al. (2014) argued that the role of NT in climate change mitigation is overestimated by at least one order of magnitude if applying a modest annual carbon accumulation rate (0.3 Mg C ha1) to the global area under cereal crops. In addition, after averaging results from pairwise plot experiments on various types of crop reported in 610 papers from 63 countries, Pittelkow et al. (2014) concluded that potential contributions of no-tillage soil management to sustainable intensification of agriculture and food security are more limited than often assumed, overall leading to a 5.7% decline of global crop yields. These recent negative assessments of NT benefits are based on averaging yields from plot and field studies (Pittelkow et al., 2014; Powlson et al., 2014). These reports point out that the effects of NT vary with climate. Consequently, a diagnosis of the regional ecology and farming system is required. However, interpretation of globally averaged values does not reflect the potential sum of regional benefits of NT appropriately, because plot and field results have not been weighted by the geographic area individual studies
apply to (e.g., in Pittelkow et al. (2014)). In addition, assessing the full benefits of NT requires an understanding of all impacts for each agro-eco-region, including future trends of climate change, soil quality changes on crop production and food security. This paper attempts such a holistic review of the benefits of conservation tillage on the Chinese Loess Plateau, including SOC, crop yields and their variability, as well as an outlook on future soil quality, to assess the need for region-specific assessments of the benefits of conservation tillage. The Chinese Loess Plateau, located in the mid-upper reaches of the Yellow River catchment (Fig. 1), covers an area of 0.63 million km2 (Tang and Nan, 2012). Eighty percent of the 0.17 million km2 of cropland on the Loess Plateau (corresponding to 10% of US cropland) are used for small-scale rainfed wheat, corn, and millet production for regional or national consumption (Ostwald and Chen, 2006). While still a regional study, the Loess Plateau has a national relevance for food security in China where the number of undernourished people declined from 21% in 1990 to 12% in 2013 (FAO, 2013). However, the limited amount of arable land and scarce water supplies will remain a challenge for further improvements. The average yields on the Loess Plateau are 3.16 t ha1 for wheat (corresponding to 60% of US crop yields) (Zhang and Liu, 2005). However, after centuries of deforestation of the hilly landscape, over-grazing, low vegetation cover, highly erodible loess soil, and heavy rain storms in summer, the Loess Plateau has become one of the most severely eroded areas in the world (Zhang and Liu, 2005; Hessel et al., 2003; He et al., 2004; Deng et al., 2010). In addition, the Loess Plateau has been exposed to a steadily increasing temperature as well as highly variable, but slightly decreasing
Fig. 1. The map of China, with the Loess Plateau delineated in bright yellow color. Map source: Australian Center for International Agricultural Research Project: Regional impacts of re-vegetation on water resources of the Loess Plateau, China and the Middle and Upper Murrumbidgee Catchment, Australia.
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annual rainfall since 1970 (Ostwald and Chen, 2006). Hence, regional agriculture is greatly dependent on and sensitive to climate change (Liu, 1999), and the stability of crop yields is essential to maintain resilience to famine in the region (Cavatassi et al., 2011). Conservation tillage on the Loess Plateau, therefore, is one of the management practices considered to protect soils, enhance yields, reduce net greenhouse gases emissions (Li et al., 2011; Wang et al., 2007), as well as increase SOC sequestration by an estimated 6 Mt C per year (Tang and Nan, 2012). This paper, based on an investigation of almost 20 years of conservation tillage plot experiments across the Loess Plateau, will assess the regional potential of conservation tillage on crop yield, SOC sequestration and its long-term benefits for mitigating climate change. Given the general understanding of the benefits of NT and the specific agro-eco conditions on the Loess Plateau discussed above, we hypothesize that: (1) NT tends to increase or at least limit the decline of crop yield, particularly in dry years, dry regions and on degrading soils; (2) NT has the potential to increase SOC stocks, yet such benefits should decrease with soil depth and cultivation time. The paper will also compare the regional observations on the Loess Plateau with the global averaging in other reports, to detect the risks of masking the sum of regional benefits by ignoring the geographic extent individual studies apply to. 2. Methods 2.1. Data sources In total, 23 papers on conservation tillage experiments on the Chinese Loess Plateau were reviewed in this study. Only twelve of those (listed in Appendix A and B) enabled a direct comparison of two tillage practices on rainfed fields over a longer period of time:
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conventional tillage with straw removal and plowing (CT), and notillage with straw retention (NT). Unlike the variety of crop types investigated by Pittelkow et al. (2014), which may generate a bias by the weight of individual crops, this study strictly focused on the differences between CT and NT on wheat production. When comparing different studies, all the data were converted into tons per hectare, both for wheat yield and SOC stocks, providing an easy way to compare and interpret the results. Information on location, precipitation, soil properties, and crop yield are listed in Appendix A and B. 2.2. Organic carbon stocks The changes of SOC stocks were calculated by comparing the total SOC stocks of all soil layers in NT to that of CT over cultivation years. The total SOC stocks were calculated by: Xn SOCi BDi hi ð1Þ SOCstocks ¼ i where SOCstocks represent the SOC stocks of the entire soil profile reported in a study with a certain tillage practice after a certain number of tillage years (t ha1); SOCi is the SOC concentration for soil depth of i (%); BDi is the bulk density for soil depth of i (g cm3); hi is the depth for soil depth of i (cm). The average annual change in SOC stocks was then calculated by normalizing the total change of SOC stocks in NT compared to CT over the tillage years on individual study plots into yearly change of SOC stocks, with a measurement unit of t ha1 a1. 2.3. Precipitation Annual precipitation data was either directly obtained from the reviewed literature, or provided by local meteorology departments. The average annual precipitation in the area of the plot
Fig. 2. The yields from conventional tillage (CT) and no-tillage (NT) under different annual precipitations. The three dotted lines indicate the convenient cut-offs drawn based on the preliminary analysis: 300 mm and 600 mm.
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studies used in this paper is 465 mm, ranging from 73 to 1182 mm between 1993 and 2007. About 65% of the total annual precipitation on the Loess Plateau occurs as rainfall during the growing season (June–September) (X. Wang et al., 2008). However, crop yield on the Loess Plateau is also restricted by soil water shortage (Li et al., 2007). As a consequence, winter drought, although outside the growing season, can exacerbate the effects of summer precipitation on crop yields (Wang et al., 2007; Huang et al., 2008). Therefore, yield risk assessment based on annual rather than summer precipitation is plausible. The annual precipitation for year 2050 and 2100 is projected from the database reported in Wan et al. (2014). 2.4. Yield analysis and spatial extrapolation The information on yields from NT and CT was directly extracted from the literature listed in Appendix A. Crop yield increases sharply once annual precipitation exceeds 300 mm, and stays rather stable when annual precipitation is above 600 mm (Fig. 2). The average crop yield for CT is about 3 t ha1, achieved around an annual precipitation of 500 mm. To assess the impact of NT, crop yields were divided by the average CT yield (ca. 3 t ha1) and separated into two groups for above and below average yields. A pairwise NT-CT comparison thus enables an assessment of NT benefits for years with drought stress, which would otherwise be masked by high yields in years with precipitation above average. To assess the spatial relevance of our plot data meta-analysis across the Loess Plateau, annual precipitation limits of 300 mm and 600 mm were chosen to delineate the spatial pattern of NT effects on increasing or stabilizing crop yields for current and future annual precipitation. Between 300 and 600 mm annual precipitation, crop yield data were further divided by increments of 100 mm to capture the sensitivity of crop yield to precipitation. Data collation was carried out using Microsoft Excel 2010, and all the figure plotting and statistical analysis were conducted using R
studio software packages (Version0.98.945). Specific statistical tests and procedures were described in individual figures and tables. 3. Results 3.1. Changes of SOC stocks across soil depths and over tillage years The benefit of NT compared to CT on the changes of SOC stocks varied across different soil depths (Fig. 3). In topsoil layers (above 20 cm), NT in general had greater SOC stocks than CT (Fig. 3), but the benefit tended to decline with soil depths, and even turned to be negative in soil layers deeper than 20 cm (Fig. 3). In addition, in each soil layer, except for the top 5 cm, the total SOC stocks generally declined with the number of years after NT adoption (Fig. 3). The average annual changes in SOC stocks on NT were considerable during the first five years after NT adoption, but tended to decrease for longer plot experiments, and even became negative in a 16-year study (Fig. 4). 3.2. Yield and precipitation The crop yield from NT was on average 14.2% larger than that from CT (Fig. 5). In years with precipitation <500 mm, NT yields were 16.2% greater than that of CT, and the most pronounced differences (+20.8%) occurred in years with annual precipitation of 400–500 mm (Fig. 5). The pairwise-comparison between the yield of no-tillage (NT) and conventional tillage (CT) for years above and below average yield in Fig. 6 enables an assessment of the effect of NT on yields in dry years. The slope of the regression line for years with yields below average is greater than for years with yields above average, confirming that the advantage of NT in increasing crop yield over CT is most pronounced in dry years or dry regions of the Loess Plateau. The area currently producing just an average yield or less, limited by an annual precipitation of 500 mm, is likely
Fig. 3. Differences of total SOC stocks, as no-tillage (NT) compared to conventional tillage (CT), in different soil depths over different cultivation years.
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Fig. 4. Average annual change of soil organic carbon (SOC) stocks in the upper 30 cm from no-tillage (NT) compared to conventional tillage (CT). The positive values of the colored bars represent the additional increase of SOC stocks in NT compared to CT, whilst the negative value of the colored bar represent the depleted amount of SOC stocks in NT compared to CT. The two dashed lines indicate the range of annual SOC sequestration rates in warm dry and cold dry regions on the Loess Plateau modeled by Tang and Nan (2012). NOTE: in the cultivation year of 15 (marked with*), the data was only available for the layer 0–20 cm.
to increase until 2100 by 31% (Fig. 7), highlighting the potential benefits of NT on the Loess Plateau. 4. Discussion 4.1. Potential to mitigate climate change The decreasing differences of total SOC stocks between NT and CT across the soil profiles on the Loess Plateau (Fig. 3) suggest that the benefits of NT to sequester atmospheric carbon are limited to
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the surface where soils are still subject to direct seeding and dense root development. This can primarily be attributed to two reasons: (1) the lack of tillage operations in NT, especially plowing, inhibits the incorporation of plant residues into deeper layers (van Oost et al., 2004); and (2) compacted soil layers are not loosened by tillage operations, which inhibits microbial activity and root growth in deep layers (Baker et al., 2007). The SOC distribution with depth observed on the Loess Plateau confirms the global review by Powlson et al. (2014). They proposed that the apparent increase in SOC under no-tillage should not be considered as a net increase in SOC stocks, as larger stocks in the surface 20 cm compared with conventional tillage are probably counteracted by declining quantities in the 20–40 cm layer under no-tillage. Baker et al. (2007) also argued that the extensively reported benefits of NT in SOC sequestration may simply be an artifact of sampling, as most results that reported SOC gains were based only on nearsurface samples which neglected the possible reduction of SOC stocks in deeper layers of soils managed by NT. The generally declining differences of total SOC stocks in each soil layer over time (Fig. 3), as well as the decreasing annual change in SOC stocks over time (Fig. 4), suggest that the NT-induced SOC change approaches its maximum capacity after about 10 years on the Loess Plateau. This finding is consistent with the sigmoid pattern proposed by West and Post (2002), and Lal (2004a) who found that the rate of SOC increase attains the maximum effect 5– 20 years after adoption of management practices aimed at increasing SOC stocks. The temporal trend also implies that linearly extrapolating annual SOC change derived from short-term observations into a long-term estimation of the overall potential of NT for improving SOC stocks carries a risk of overestimation. For example, Tang and Nan (2012) estimated that no-tillage and high residue incorporation techniques have the potential to increase
Fig. 5. The relative difference of yield in no-till (NT) vs. conventional tillage (CT) systems on the Chinese Loess Plateau. This figure is based on data reported in references listed in Appendix A. Yields were categorized by annual precipitation, and the number of observations in each category is indicated in the brackets. Bars in boxes represent median values. Whiskers indicate the lowest datum within 1.5 interquartile range of the lower quartile, and the highest datum within 1.5 interquartile range of the upper quartile.
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Fig. 6. The pairwise-comparison between the yield of no-tillage (NT) and conventional tillage (CT) (t-test, at significance level of p 0.05). Filled dots represent pairs of yield below average CT yield (n = 30), while open dots represent pairs of yield above average CT yield (n = 35). Dashed line denotes the regression line of yields below average CT yield, while dotted line denote the regression line of yields above average CT yield. Bold line in the middle represents the 1:1 ratio.
SOC sequestration by up to 0.4 t ha1 per year on the Loess Plateau. However, with reduced or no-tillage, less litter is moved from the soil surface deeper into soil profile (Powlson et al., 2014; van Oost et al., 2004). Hence, the increased SOC concentration is very likely to be constrained to topsoil layers that are still mixed by direct seeding (Liu et al., 2014), and balanced out by a decline in the soil profile not subject to tillage anymore. Furthermore, Liu et al. (2014) reported that the increase of SOC stocks is positively correlated with labile organic carbon under NT in northern China. In their study, microbial biomass was significantly increased under NT as well. Similarly, Q. Wang et al. (2008) also detected greater microbial biomass carbon and higher respiration rates per unit soil on the NT fields on the Loess Plateau. This implies that the newly formed SOC may have not been converted into stable organic matter. While other long-term benefits of NT have been well recognized, particularly in terms of increasing resistance to soil erosion, improving water use efficiency, and reducing labor and energy inputs (Pimentel et al., 1995; Leys et al., 2007; Tullberg et al., 2007; Knapen et al., 2008; Bai et al., 2009; He et al., 2010a; Li et al., 2011; Bhattacharyya et al., 2012), ignoring the liability/ stability of newly formed SOC questions the commonly used linear extrapolations on the overall storage of SOC in the soil profile and climate change mitigation attributed to conservation tillage (Li et al., 2012). 4.2. Contribution to ensure food security Unlike SOC stocks, the effect of NT on crop yields was positive in the Loess Plateau studies. The average crop yield increase for NT (+14.2%) observed in the plot studies (Fig. 5) is in strong contrast to the 5.7% decline found by Pittelkow et al. (2014) in their global assessment. More importantly, during years with annual
precipitation <500 mm, NT yields were 16.2% greater than CT yields on the Loess Plateau. Similar effects on yield increase from other conservation tillage practices on the Loess Plateau or arid northwest China have also been reported (He et al., 2010b; Bai et al., 2009; Wang et al., 2007; He et al., 2007a), as well on other crops, for instance, pea (Huang et al., 2008), maize (He et al., 2007b), millet and naked oats (He et al., 2010b). Such yield increase reduces the probability of food production falling below minimum thresholds of subsistence requirements, thereby increasing resilience to famine (Cavatassi et al., 2011). This is even more remarkable when considering that the area with marginal annual precipitation (<500 mm) for crop production on the Loess Plateau is likely to increase during the next 100 years by 31% (Fig. 7). Apart from the average and future yields, inter-annual variability also has to be considered. NT drought resilience on the Loess Plateau, illustrated by the greater yield improvements during years with below average precipitation, shows that long-term averages do not distinguish between sustainable intensification of agriculture and food security in dry years. However, the latter is equally relevant because famine is not caused by lack of global average production, but regional access to affordable food after poor harvests (Cavatassi et al., 2011). Increasing regional resilience to drought is therefore an essential NT benefit, especially for poor farmers. Finally, comparing yields and SOC stocks between plot treatments does not fully reflect the future feedback of NT on soil protection and productivity. For instance, on the Loess Plateau, up to 70% of arable land is affected by an annual soil loss of 20–25 t ha1 (Shi and Shao, 2000), far exceeding the threshold for sustainable use (e.g. 10 t ha1 proposed by Doetterl et al. (2012)). Such non-sustainable soil loss is not limited to the Loess Plateau, but affects 42% of cropland worldwide (Kuhn, 2014). If NT reduces erosion rates to avoid soil destruction (X. Wang et al., 2008), then even maintaining
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(2014). However, the positive effect was masked in their global assessment by an overall negative NT average on the 610 plots. The papers used by Pittelkow et al. (2014) rely mainly on data from the American East and Midwest, Europe, India and eastern China, out of which about 30% is wheat, 27% is maize, and the rest 43% from other types of crop. An overlay of their study locations and the areas classified as drylands by FAO (2008) show that 42% of their study sites are located in drylands (Appendix C). Yet, only approximately 30% of global croplands are situated in drylands (FAO, 2008), suggesting an over-representation of drylands by the plot studies in Pittelkow et al. (2014). This means that, as a result of global averaging, the negative results of NT from the underrepresented humid regions still outweighed the positive effects from the over-represented drylands. In addition, globally, 66% of wheat area and 82% of maize area is rainfed (Venkateswarlu and Shanker, 2012), demanding much more weight than the 30% wheat and 27% maize reflected in Pittelkow et al. (2014), if evaluating the potential effects of NT on global crop yields. The discrepancies between plot numbers and the extent of the areas they apply to, illustrate the risks that global averaging masks the benefits of NT in the context of a particular combination of eco-region and tillage practice (please also see our comment online following Pittelkow et al. (2014)). In particular for large regions such as the Loess Plateau, with croplands corresponding to 6.4% of all cropland in drylands (2.12 million km2 according to FAO (2008)), global averaging carries the risk of disregarding beneficial soil management practices such as NT in policy development aimed at achieving sustainable regional and national food production. 5. Conclusion
Fig. 7. The spatial distribution of mean annual precipitation in the Loess Plateau region in 2010, and the projected spatial distribution of mean annual precipitation in 2050 and 2100. The GIS data was adopted from Wan et al., (2014). The highlighted areas represent the areas, where no-tillage has shown the most pronounced advantage in increasing yield under current climate conditions. The six cities are the locations of the experimental sites from the reviewed literature listed in Appendix A.
yields and SOC stocks, without increasing the use of mineral fertilizer and the associated emissions, represent a significant contribution to sustainable food production. 4.3. Assessing NT benefits by agro-ecosystem The positive impact of NT on the Loess Plateau corresponds to the NT benefits in drylands acknowledged by Pittelkow et al.
This paper reviewed almost 20 years of NT plot experiments across the Chinese Loess Plateau, aiming to assess the long-term benefits of NT for mitigating and adapting to climate change, and thus contribute to the sustainable intensification of agriculture. Furthermore, the risks associated with global averaging of plot data were analyzed. The observed decreasing total SOC stocks changes with soil depth suggest that benefits of NT in sequestrating SOC are only concentrated to the immediate topsoil still subject to direct seeding. The total SOC stocks and the annual change in SOC stocks also declined over time, suggesting that the soil carbon sink reaches a maximum capacity after about 10 years of NT on the Loess Plateau. The temporal trend of SOC stocks observed in this study confirms some recent papers (Liu et al., 2014; Powlson et al., 2014), but contradicts the statement by Tang and Nan (2012) on the potential of NT on the Loess Plateau to mitigate climate change. As a consequence, the overall potential of NT in improving SOC stocks on the Loess Plateau is very likely to be over-estimated, if linearly extrapolating annual change derived from short-term observation to long-term estimation. The contrasting conclusions on NT crop yield derived from this study for the Chinese Loess Plateau, the FAO reports (FAO, 2011, 2008) and Pittelkow et al. (2014) illustrate that the contribution of NT to sustainable food production and food security is not yet fully understood. A key reason for the different results appears to be the global averaging of pairwise plot studies where all plots are given equal weighting, irrespective of the area they would apply to and the regional, national or international relevance of food production in this area. Instead of global averaging, the spatially weighted sum of impacts for each combination of eco-region and tillage practice, including future trends of climate change and soil quality on crop production and food security, should be considered when studying NT. Clearly, policy decisions should be based on agro-eco-region analysis, rather than global averaging, and should also include factors such as tillage system performance in dry years, especially the impact on future yields, soil loss and thus productivity
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decrease. The different interpretations of NT performance of global and regional meta-analysis also demonstrate that NT impacts are far from being adequately understood, but have to be seen as research in progress.
for sharing his GIS database, and the contribution of Samuel Kuonen to process the GIS database. Appendix A. See Table A1.
Acknowledgement The authors gratefully acknowledge Liangang Xiao for collating the data from published papers. We are grateful to Xiaoping Zhang
Appendix B. See Table B1.
Table A1 Information of the locations, cultivation years, crop yields, precipitation, water use efficiency of conventional tillage (CT) and no-tillage (NT) collected from the studies on the Chinese Loess Plateau investigated in this paper.
Chen et al. (2008)
He et al. (2009)
Huang et al. (2008)
Jin et al. (2007)
Jin et al. (2009)
Li et al. (2007)
Su et al. (2007)
Journal
Study location
Cultivation years
Crops
Australian Journal of Soil Research, 2008, 46, 645–651
Linfen, Shanxi (37 320 –38 60 N, 112 40 –113 260 E)
8
Wheat No
1999 422
Wheat No
2000 2001 2002 2003 2004 2005 2006 1998
Soil Use and Management, June Wuchuan, Inner Mongolia 2009, 25, 201–209 (38 6 N, 113 E)
Field Crops Research 107 (2008), Dingxi, Gansu (35 28 N, 43–55 104 44E)
Soil & Tillage Research 96 (2007), 131–144
Soil & Tillage Research 104 (2009), 115–120
Australian Journal of Soil Research, 2007, 45, 344–350
Luoyang, Henan (34 50 N, 113 E)
Luoyang, Henan (34 50 N, 113 E)
Linfeng, Shanxi (37 320 –38 60 N, 112 40 –113 260 E)
Agricultural Water Management Luoyang, Henan (34 50 N, 87 (2007), 307–314 113 E)
10
4
5
7
15
6
Irrigation Year
Annual precipitation (mm)
Average yield (t ha1)
Water use efficiency (t ha1 mm1)
CT
NT
CT
NT
3.79
3.27
17.2
15.1
328 443 418 447 669 308 424 159.1
1.46 2.91 3.52 3.64 4.12 1.91 3.5 1.071
2.48 3.08 3.68 3.51 4.01 2.71 4.43 1.181
11.8 9.5 10.9 9.7 10.1 10.3 NA 4.5
12.6 9.7 14 10.5 9.1 11.1 NA 4.9
Wheat No
1999 2000 2001 2002 2003 2004 2005 2006 2007 2002
190 274.9 268.9 254.9 167.2 309.7 286.4 206.1 192.6 351
1.142 1.365 1.234 1.321 1.057 1.486 1.289 1.116 1.104 2.1
1.324 1.42 1.348 1.478 1.326 1.511 1.511 1.426 1.365 2.3
4.3 4.2 3.8 4.2 4.4 4.2 3.6 4.1 4.2 7.1
5.1 4.3 4.2 4.8 5.4 4.2 4.1 5.2 4.9 7.8
Wheat No
2003 2004 2005 2001
565 345 453 775
1.5 2.2 2.6 3.922
2 2.3 3.1 4.107
4.7 9.1 8.8 0.53
6.1 8.5 9.7 0.54
Wheat No
2002 2003 2004 2005 2000
742 521 1182 724 573
5.169 4.019 4.347 4.801 4.218
4.897 4.778 4.27 5.292 3.856
0.71 0.7 0.38 0.67 NA
0.67 0.83 0.38 0.74 NA
Wheat No
2001 2002 2003 2004 2005 2006 1993
521 512 1038 707 586 NA 470
3.922 5.169 4.019 4.347 4.801 5.171 2.548
4.107 4.897 4.778 4.27 5.292 5.383 2.985
NA NA NA NA NA NA 9.4
NA NA NA NA NA NA 11.2
Wheat No
1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2000
522 434 535 574 359 422 328 443 418 447 669 308 424 573
3.002 2.342 3.455 3.908 2.495 2.148 1.652 2.917 3 3.36 4.605 2.65 4.41 3.857
3.162 2.513 3.865 4.142 3.06 2.644 2.578 3.814 4.23 3.975 5.175 3.765 4.696 4.218
12.3 9.1 19.1 15.3 10.2 15.6 13.8 13.4 17.2 16.4 14.4 17.4 17.5 1.205
13 9.8 22.8 16.3 11.5 18.2 19.4 16.9 23.1 19.6 16.1 20.1 18.4 1.3
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163
Table A1 (Continued) Journal
Wang et al. Soil & Tillage Research 104 (2009) (2009), 192–197
Zhang et al. Soil & Tillage Research 102 (2009) (2009), 78–86
Study location
Linfeng, Shanxi (37 320 –38 60 N, 112 40 –113 260 E)
Heyang, Shanxi (35190 N, 110 40 E)
Cultivation years
6
3
Annual precipitation (mm)
Average yield (t ha1)
Water use efficiency (t ha1 mm1)
CT
NT
CT
NT
Wheat No
2001 2002 2003 2004 2005 1999
521 512 1038 707 586 422
4.021 4.469 3.541 4.741 4.118 3.79
4.607 5.007 4.355 5.225 4.662 3.27
1.112 1.106 1.106 1.004 1.17 17.2
1.137 1.201 1.201 1.162 1.291 15.1
Wheat No
2000 2001 2002 2003 2004 2005 2002
328 443 418 447 669 308 513.159
1.46 2.91 3.52 3.64 4.12 1.91 2.63
2.48 3.08 3.68 3.51 4.01 2.71 2.91
11.8 9.5 10.9 9.7 10.1 10.3 NA
12.6 9.7 14 10.5 9.1 11.1 NA
2.57 4.8
3.01 4.51
NA NA
NA NA
Crops
Irrigation Year
2003 435.3 2004 998.6
Table B1 Information of the locations, cultivation years, soil organic carbon stocks in different depths of conventional tillage (CT) and no-tillage (NT) collected from the studies on the Chinese Loess Plateau investigated in this paper. Journal
Location
Years Crop type
Irrigation Soil organic carbon stocks (t ha1) Soil depth (cm)
Chen et al. (2008)
Chen et al. (2009)
He et al. (2009)*
Jin et al. (2007)
Li et al. (2007)*
Liu et al. (2014)
Wang et al. (2008)
Australian Journal of Soil Research, 2008, 46, 645–651
Soil & Tillage Research 106 (2009), 85–94
New Zealand Journal of Crop and Horticultural Science, 2009, vol. 37: 157–166
Soil & Tillage Research 104 (2009), 115–120
Australian Journal of Soil Research, 2007, 45, 344–350
Geoderma 213 (2014), 379–384
Geoderma 144 (2008), 502–508
Linfen, Shanxi (37 320 –38 60 N, 112 40 –113 260 E)
Linfen, Shanxi (38 60 N, 1130 E)
11
16
Linfen, Shanxi (38 60 N, 113 E)
Luoyang, Henan (34 50 N, 113 E)
0
0
Linfeng, Shanxi (37 32 –38 6 N, 112 40 –113 260 E)
Linfeng, Shanxi (37 320 –38 60 N, 112 40 –113 260 E)
Linfen, Shanxi (38 60 N, 113 E)
8
5
15
17
16
Wheat
Wheat
Wheat
Wheat
Wheat
Wheat
Wheat
CT
No
0–10
No
10–20 20–30 total 0–15
7.24 7.98 3.51 4.81 20.65 26.03 19.6 22.2
No
15–30 Total 0–5
20.2 15.4 39.8 37.6 9.34 12.77
No
5–10 10–20 20–30 total 0–20
10.40 11.68 20.01 16.32 14.17 9.01 53.93 49.78 20.3 23.86
No
20–30 total 0–10
7.51 27.78 17.76
No
20–20 total 0–5
12.98 15.75 30.75 39.61 4.5 9.5
No
5–10 10–20 20–30 30–40 40–50 50–60 0–5
3.9 6.2 9.3 9.7 8.5 7.8 7.7 5.6 6.5 5.9 5.8 5.4 9.34 12.77
5–10 10–20 20–30 Total
10.40 11.68 20.01 16.32 14.17 9.01 53.93 49.78
NOTE: Soil organic carbon (SOC) stocks in the papers marked with * was calculated from soil organic matter content by multiplying a factor of 1.73.
9.91
NT 13.23
8.57 32.42 23.87
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Fig. C1. An overlay of the study locations in Pittelkow et al. (2014) and the areas classified as drylands by the FAO (2008) report. Green dots indicate the study locations inside drylands, whereas the red dots indicate the study locations outside drylands. Study locations, and aridity information were extracted from the supplement materials of Pittelkow et al. (2014).
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N. Y. Acad. Sci. 1195, E96–106. doi:http://dx.doi.org/10.1111/j.17496632.2009.05402.x. Hessel, R., Messing, I., Liding, C., Ritsema, C., Stolte, J., 2003. Soil erosion simulations of land use scenarios for a small Loess Plateau catchment. CATENA 54, 289–302. doi:http://dx.doi.org/10.1016/s0341-8162(03) 00070-5. He, X., Tang, K., Zhang, X., 2004. Soil erosion dynamics on the Chinese Loess Plateau in the last 10,000 years. Mt. Res. Dev. 24, 342–347. Huang, G.B., Zhang, R.Z., Li, G.D., Li, L.L., Chan, K.Y., Heenan, D.P., Chen, W., Unkovich, M.J., Robertson, M.J., Cullis, B.R., Bellotti, W.D., 2008. Productivity and sustainability of a spring wheat-field pea rotation in a semi-arid environment under conventional and conservation tillage systems. Field Crops Res. 107, 43– 55. doi:http://dx.doi.org/10.1016/j.fcr.2007.12.011. Ipcc, (2013). Climate Change 2013: The Physical Science Basis: Working Group I Contribution to the Fifth Assessment Report of the Intergovernmental Panel on Climate Change, Stocker T.F., D. Qin, G.-K. Plattner, M. Tignor, S.K. Allen, J. Boschung, A. Nauels, Y. Xia, V. Bex and P.M. Midgley (eds.). Cambridge University Press, Cambridge, United Kingdom and New York, USA. Knapen, A., Poesen, J., Govers, G., De Baets, S., 2008. The effect of conservation tillage on runoff erosivity and soil erodibility during concentrated flow. Hydrol. Process. 22, 1497–1508. doi:http://dx.doi.org/10.1002/hyp.6702. Kuhn N.J., (2014). Soil Loss, in: The Soil Underfoot: Infinite Possibilities for a Finite Resource. eds. Churchman G.J. & Landa E.R., pp. 37–48. Lal, R., 2004a. Soil carbon sequestration impacts on global climate change and food security. Science 304, 1623–1627. doi:http://dx.doi.org/10.1126/ science.1097396. Lal, R., 2004b. Carbon emission from farm operations. Environ. Int. 30, 981–990. doi: http://dx.doi.org/10.1016/j.envint.2004.03.005. Leys, A., Govers, G., Gillijns, K., Poesen, J., 2007. Conservation tillage on loamy soils: explaining the variability in interrill runoff and erosion reduction. Eur. J. Soil Sci. 58, 1425–1436. doi:http://dx.doi.org/10.1111/j.1365-2389.2007.00947.x. Li, C., Yue, L., Kou, Z., Zhang, Z., Wang, J., Cao, C., 2012. Short-term effects of conservation management practices on soil labile organic carbon fractions under a rape-rice rotation in central China. Soil Tillage Res. 119, 31–37. doi: http://dx.doi.org/10.1016/j.still.2011.12.005. Li, H., Gao, H., Wu, H., Li, W., Wang, X., He, J., 2007. Effects of 15 years of conservation tillage on soil structure and productivity of wheat cultivation in northern China. Aust. J. Soil Res. 45, 344. doi:http://dx.doi.org/10.1071/SR07003. Li, L., Huang, G., Zhang, R., Bill, B., Guangdi, L., Kwong, Y.C., 2011. Benefits of conservation agriculture on soil and water conservation and its progress in China. Agric. Sci. China 10, 850–859. doi:http://dx.doi.org/10.1016/s1671-2927 (11) 60071-0. Liu, E., Teclemariam, S.G., Yan, C., Yu, J., Gu, R., Liu, S., He, W., Liu, Q., 2014. Long-term effects of no-tillage management practice on soil organic carbon and its fractions in the northern China. Geoderma 213, 379–384. doi:http://dx.doi.org/ 10.1016/j.geoderma.2013.08.021. Liu, G., 1999. Soil conservation and sustainable agriculture on the Loess Plateau: challenges and prospects. Ambio 663–668. Ostwald, M., Chen, D., 2006. Land-use change: Impacts of climate variations and policies among small-scale farmers in the Loess Plateau, China. Land Use Policy 23, 361–371. doi:http://dx.doi.org/10.1016/j.landusepol.2005.04.004.
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Conservation tillage and sustainable intensification of agriculture: regional vs. global benefit analysis Nikolaus J. Kuhna , Yaxian Hua,* , Lena Bloemertza , Jin Heb , Hongwen Lib , Philip Greenwooda a b
Physical Geography and Environmental Change Research Group, University of Basel, Klingelbergstrasse 27, Basel 4056, Switzerland Conservation Tillage Research Center, China Agricultural University, Qinghua East Road 17, Beijing 100083, China
A R T I C L E I N F O
A B S T R A C T
Article history: Received 20 June 2015 Received in revised form 30 September 2015 Accepted 1 October 2015 Available online xxx
Climate change is expected to affect both the amount of global crop production, and annual variability in food supply. Agriculture is a major source of greenhouse gas emissions, but also considered to mitigate climate change. Conservation tillage, as a climate-smart agricultural practice, is repeatedly reported to mitigate net greenhouse gas emissions by increasing soil organic carbon (SOC). However, with reduced tillage, less litter is moved from the surface deeper into the soil profile, so SOC increase is very likely constrained to topsoil layers. Further adaptation benefits, such as increasing crop yield and resilience to famine, have recently been questioned after averaging yields from field studies. However, such global averaging masks the geographic extent individual studies apply to. This paper attempts a holistic regional analysis on the benefits of conservation tillage, in particular its fundamental principle no-tillage (NT), on the Chinese Loess Plateau. Based on a review of almost 20 years of conservation tillage plot experiments, the potential of NT to increase SOC stocks and to adapt to lower but more variable rainfall in the future has been assessed. The results show that the difference of total SOC stocks between NT and CT decreased with soil depth, confirming that the SOC benefits of NT are concentrated to the immediate topsoil still subject to direct seeding. The topsoil achieved maximum SOC stocks after about 10 years of NT. Crop yields from NT increased by up to 20% for years with average and below average precipitation, demonstrating the advantages of NT in stabilizing crop yields in dry years. However, the results in previous reports are not weighted by the actual spatial extent of drylands and humid regions after counting individual plot studies. As a consequence of such global and unweighted averaging, the benefits from NT to increase SOC stocks are likely to misrepresent the actual impact. Therefore, given the size of the Loess Plateau and its relevance for food security in China, our analysis illustrates the need to assess the benefits of a tillage and residue management system for each combination of eco-region and farming practice, weighted by their area and the affected population, rather than just using a global average for policy development on sustainable productivity. ã 2015 Elsevier B.V. All rights reserved.
Keywords: Conservation tillage Carbon sequestration Food security Global averaging Spatial weighting
1. Introduction Climate change is expected to affect global food security by changing both spatial and temporal patterns of yields (Wheeler and von Braun, 2013). Agriculture also causes emissions of 500 Tg C to the atmosphere per year (Lal, 2004a). In particular, the use of nitrogen fertilizers contributes 187–224 Tg CH4 and 1.7–4.8 Tg N2O per year (Ipcc, 2013). Meanwhile, agriculture could also mitigate climate change (Vermeulen et al., 2012), most notably by enhancing soil organic carbon (SOC) on degraded agricultural
* Corresponding author. E-mail address: [email protected] (Y. Hu). http://dx.doi.org/10.1016/j.agee.2015.10.001 0167-8809/ ã 2015 Elsevier B.V. All rights reserved.
lands. The contribution is equivalent to 50–60% of the 42–78 Pg C of soil carbon released from human-induced land use change over 25–50 years (Ipcc, 2013). In particular, conservation tillage, as a manner of climate-smart agriculture practice, is repeatedly reported to mitigate climate change by reducing net greenhouse gas emissions (Lal, 2004b; X. Wang et al., 2008; He et al., 2010b; Li et al., 2012). Conservation tillage may also contribute to climate change adaptation by increasing or at least limit the decline of crop yields in dry years (FAO, 2011), and thus help to strengthen resilience to famine by ensuring regional access to affordable food in dry years/dry regions (Cavatassi et al., 2011). Conservation tillage also has co-benefits that include increased resistance to soil erosion (Pimentel et al., 1995; Leys et al., 2007; Knapen et al., 2008), improving water use efficiency (Bai et al., 2009; He et al.,
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2010a; Li et al., 2011), and reducing labor and energy inputs (Tullberg et al., 2007). A fundamental principle of conservation tillage is no-tillage (NT), which minimizes mechanical disturbance of soil, limiting macro-aggregate destruction, and thereby reducing exposure of soil organic matter to mineralization (Rhoton et al., 2002; Six et al., 2004; Liu et al., 2014). While conservation tillage is in general considered to help increase crop yields and sequestrate atmospheric SOC (as discussed above), some recent papers argued that the potential contribution of NT to climate change mitigation and food security is more limited than initially thought. For instance, Powlson et al. (2014) argued that the role of NT in climate change mitigation is overestimated by at least one order of magnitude if applying a modest annual carbon accumulation rate (0.3 Mg C ha1) to the global area under cereal crops. In addition, after averaging results from pairwise plot experiments on various types of crop reported in 610 papers from 63 countries, Pittelkow et al. (2014) concluded that potential contributions of no-tillage soil management to sustainable intensification of agriculture and food security are more limited than often assumed, overall leading to a 5.7% decline of global crop yields. These recent negative assessments of NT benefits are based on averaging yields from plot and field studies (Pittelkow et al., 2014; Powlson et al., 2014). These reports point out that the effects of NT vary with climate. Consequently, a diagnosis of the regional ecology and farming system is required. However, interpretation of globally averaged values does not reflect the potential sum of regional benefits of NT appropriately, because plot and field results have not been weighted by the geographic area individual studies
apply to (e.g., in Pittelkow et al. (2014)). In addition, assessing the full benefits of NT requires an understanding of all impacts for each agro-eco-region, including future trends of climate change, soil quality changes on crop production and food security. This paper attempts such a holistic review of the benefits of conservation tillage on the Chinese Loess Plateau, including SOC, crop yields and their variability, as well as an outlook on future soil quality, to assess the need for region-specific assessments of the benefits of conservation tillage. The Chinese Loess Plateau, located in the mid-upper reaches of the Yellow River catchment (Fig. 1), covers an area of 0.63 million km2 (Tang and Nan, 2012). Eighty percent of the 0.17 million km2 of cropland on the Loess Plateau (corresponding to 10% of US cropland) are used for small-scale rainfed wheat, corn, and millet production for regional or national consumption (Ostwald and Chen, 2006). While still a regional study, the Loess Plateau has a national relevance for food security in China where the number of undernourished people declined from 21% in 1990 to 12% in 2013 (FAO, 2013). However, the limited amount of arable land and scarce water supplies will remain a challenge for further improvements. The average yields on the Loess Plateau are 3.16 t ha1 for wheat (corresponding to 60% of US crop yields) (Zhang and Liu, 2005). However, after centuries of deforestation of the hilly landscape, over-grazing, low vegetation cover, highly erodible loess soil, and heavy rain storms in summer, the Loess Plateau has become one of the most severely eroded areas in the world (Zhang and Liu, 2005; Hessel et al., 2003; He et al., 2004; Deng et al., 2010). In addition, the Loess Plateau has been exposed to a steadily increasing temperature as well as highly variable, but slightly decreasing
Fig. 1. The map of China, with the Loess Plateau delineated in bright yellow color. Map source: Australian Center for International Agricultural Research Project: Regional impacts of re-vegetation on water resources of the Loess Plateau, China and the Middle and Upper Murrumbidgee Catchment, Australia.
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annual rainfall since 1970 (Ostwald and Chen, 2006). Hence, regional agriculture is greatly dependent on and sensitive to climate change (Liu, 1999), and the stability of crop yields is essential to maintain resilience to famine in the region (Cavatassi et al., 2011). Conservation tillage on the Loess Plateau, therefore, is one of the management practices considered to protect soils, enhance yields, reduce net greenhouse gases emissions (Li et al., 2011; Wang et al., 2007), as well as increase SOC sequestration by an estimated 6 Mt C per year (Tang and Nan, 2012). This paper, based on an investigation of almost 20 years of conservation tillage plot experiments across the Loess Plateau, will assess the regional potential of conservation tillage on crop yield, SOC sequestration and its long-term benefits for mitigating climate change. Given the general understanding of the benefits of NT and the specific agro-eco conditions on the Loess Plateau discussed above, we hypothesize that: (1) NT tends to increase or at least limit the decline of crop yield, particularly in dry years, dry regions and on degrading soils; (2) NT has the potential to increase SOC stocks, yet such benefits should decrease with soil depth and cultivation time. The paper will also compare the regional observations on the Loess Plateau with the global averaging in other reports, to detect the risks of masking the sum of regional benefits by ignoring the geographic extent individual studies apply to. 2. Methods 2.1. Data sources In total, 23 papers on conservation tillage experiments on the Chinese Loess Plateau were reviewed in this study. Only twelve of those (listed in Appendix A and B) enabled a direct comparison of two tillage practices on rainfed fields over a longer period of time:
157
conventional tillage with straw removal and plowing (CT), and notillage with straw retention (NT). Unlike the variety of crop types investigated by Pittelkow et al. (2014), which may generate a bias by the weight of individual crops, this study strictly focused on the differences between CT and NT on wheat production. When comparing different studies, all the data were converted into tons per hectare, both for wheat yield and SOC stocks, providing an easy way to compare and interpret the results. Information on location, precipitation, soil properties, and crop yield are listed in Appendix A and B. 2.2. Organic carbon stocks The changes of SOC stocks were calculated by comparing the total SOC stocks of all soil layers in NT to that of CT over cultivation years. The total SOC stocks were calculated by: Xn SOCi BDi hi ð1Þ SOCstocks ¼ i where SOCstocks represent the SOC stocks of the entire soil profile reported in a study with a certain tillage practice after a certain number of tillage years (t ha1); SOCi is the SOC concentration for soil depth of i (%); BDi is the bulk density for soil depth of i (g cm3); hi is the depth for soil depth of i (cm). The average annual change in SOC stocks was then calculated by normalizing the total change of SOC stocks in NT compared to CT over the tillage years on individual study plots into yearly change of SOC stocks, with a measurement unit of t ha1 a1. 2.3. Precipitation Annual precipitation data was either directly obtained from the reviewed literature, or provided by local meteorology departments. The average annual precipitation in the area of the plot
Fig. 2. The yields from conventional tillage (CT) and no-tillage (NT) under different annual precipitations. The three dotted lines indicate the convenient cut-offs drawn based on the preliminary analysis: 300 mm and 600 mm.
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studies used in this paper is 465 mm, ranging from 73 to 1182 mm between 1993 and 2007. About 65% of the total annual precipitation on the Loess Plateau occurs as rainfall during the growing season (June–September) (X. Wang et al., 2008). However, crop yield on the Loess Plateau is also restricted by soil water shortage (Li et al., 2007). As a consequence, winter drought, although outside the growing season, can exacerbate the effects of summer precipitation on crop yields (Wang et al., 2007; Huang et al., 2008). Therefore, yield risk assessment based on annual rather than summer precipitation is plausible. The annual precipitation for year 2050 and 2100 is projected from the database reported in Wan et al. (2014). 2.4. Yield analysis and spatial extrapolation The information on yields from NT and CT was directly extracted from the literature listed in Appendix A. Crop yield increases sharply once annual precipitation exceeds 300 mm, and stays rather stable when annual precipitation is above 600 mm (Fig. 2). The average crop yield for CT is about 3 t ha1, achieved around an annual precipitation of 500 mm. To assess the impact of NT, crop yields were divided by the average CT yield (ca. 3 t ha1) and separated into two groups for above and below average yields. A pairwise NT-CT comparison thus enables an assessment of NT benefits for years with drought stress, which would otherwise be masked by high yields in years with precipitation above average. To assess the spatial relevance of our plot data meta-analysis across the Loess Plateau, annual precipitation limits of 300 mm and 600 mm were chosen to delineate the spatial pattern of NT effects on increasing or stabilizing crop yields for current and future annual precipitation. Between 300 and 600 mm annual precipitation, crop yield data were further divided by increments of 100 mm to capture the sensitivity of crop yield to precipitation. Data collation was carried out using Microsoft Excel 2010, and all the figure plotting and statistical analysis were conducted using R
studio software packages (Version0.98.945). Specific statistical tests and procedures were described in individual figures and tables. 3. Results 3.1. Changes of SOC stocks across soil depths and over tillage years The benefit of NT compared to CT on the changes of SOC stocks varied across different soil depths (Fig. 3). In topsoil layers (above 20 cm), NT in general had greater SOC stocks than CT (Fig. 3), but the benefit tended to decline with soil depths, and even turned to be negative in soil layers deeper than 20 cm (Fig. 3). In addition, in each soil layer, except for the top 5 cm, the total SOC stocks generally declined with the number of years after NT adoption (Fig. 3). The average annual changes in SOC stocks on NT were considerable during the first five years after NT adoption, but tended to decrease for longer plot experiments, and even became negative in a 16-year study (Fig. 4). 3.2. Yield and precipitation The crop yield from NT was on average 14.2% larger than that from CT (Fig. 5). In years with precipitation <500 mm, NT yields were 16.2% greater than that of CT, and the most pronounced differences (+20.8%) occurred in years with annual precipitation of 400–500 mm (Fig. 5). The pairwise-comparison between the yield of no-tillage (NT) and conventional tillage (CT) for years above and below average yield in Fig. 6 enables an assessment of the effect of NT on yields in dry years. The slope of the regression line for years with yields below average is greater than for years with yields above average, confirming that the advantage of NT in increasing crop yield over CT is most pronounced in dry years or dry regions of the Loess Plateau. The area currently producing just an average yield or less, limited by an annual precipitation of 500 mm, is likely
Fig. 3. Differences of total SOC stocks, as no-tillage (NT) compared to conventional tillage (CT), in different soil depths over different cultivation years.
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Fig. 4. Average annual change of soil organic carbon (SOC) stocks in the upper 30 cm from no-tillage (NT) compared to conventional tillage (CT). The positive values of the colored bars represent the additional increase of SOC stocks in NT compared to CT, whilst the negative value of the colored bar represent the depleted amount of SOC stocks in NT compared to CT. The two dashed lines indicate the range of annual SOC sequestration rates in warm dry and cold dry regions on the Loess Plateau modeled by Tang and Nan (2012). NOTE: in the cultivation year of 15 (marked with*), the data was only available for the layer 0–20 cm.
to increase until 2100 by 31% (Fig. 7), highlighting the potential benefits of NT on the Loess Plateau. 4. Discussion 4.1. Potential to mitigate climate change The decreasing differences of total SOC stocks between NT and CT across the soil profiles on the Loess Plateau (Fig. 3) suggest that the benefits of NT to sequester atmospheric carbon are limited to
159
the surface where soils are still subject to direct seeding and dense root development. This can primarily be attributed to two reasons: (1) the lack of tillage operations in NT, especially plowing, inhibits the incorporation of plant residues into deeper layers (van Oost et al., 2004); and (2) compacted soil layers are not loosened by tillage operations, which inhibits microbial activity and root growth in deep layers (Baker et al., 2007). The SOC distribution with depth observed on the Loess Plateau confirms the global review by Powlson et al. (2014). They proposed that the apparent increase in SOC under no-tillage should not be considered as a net increase in SOC stocks, as larger stocks in the surface 20 cm compared with conventional tillage are probably counteracted by declining quantities in the 20–40 cm layer under no-tillage. Baker et al. (2007) also argued that the extensively reported benefits of NT in SOC sequestration may simply be an artifact of sampling, as most results that reported SOC gains were based only on nearsurface samples which neglected the possible reduction of SOC stocks in deeper layers of soils managed by NT. The generally declining differences of total SOC stocks in each soil layer over time (Fig. 3), as well as the decreasing annual change in SOC stocks over time (Fig. 4), suggest that the NT-induced SOC change approaches its maximum capacity after about 10 years on the Loess Plateau. This finding is consistent with the sigmoid pattern proposed by West and Post (2002), and Lal (2004a) who found that the rate of SOC increase attains the maximum effect 5– 20 years after adoption of management practices aimed at increasing SOC stocks. The temporal trend also implies that linearly extrapolating annual SOC change derived from short-term observations into a long-term estimation of the overall potential of NT for improving SOC stocks carries a risk of overestimation. For example, Tang and Nan (2012) estimated that no-tillage and high residue incorporation techniques have the potential to increase
Fig. 5. The relative difference of yield in no-till (NT) vs. conventional tillage (CT) systems on the Chinese Loess Plateau. This figure is based on data reported in references listed in Appendix A. Yields were categorized by annual precipitation, and the number of observations in each category is indicated in the brackets. Bars in boxes represent median values. Whiskers indicate the lowest datum within 1.5 interquartile range of the lower quartile, and the highest datum within 1.5 interquartile range of the upper quartile.
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Fig. 6. The pairwise-comparison between the yield of no-tillage (NT) and conventional tillage (CT) (t-test, at significance level of p 0.05). Filled dots represent pairs of yield below average CT yield (n = 30), while open dots represent pairs of yield above average CT yield (n = 35). Dashed line denotes the regression line of yields below average CT yield, while dotted line denote the regression line of yields above average CT yield. Bold line in the middle represents the 1:1 ratio.
SOC sequestration by up to 0.4 t ha1 per year on the Loess Plateau. However, with reduced or no-tillage, less litter is moved from the soil surface deeper into soil profile (Powlson et al., 2014; van Oost et al., 2004). Hence, the increased SOC concentration is very likely to be constrained to topsoil layers that are still mixed by direct seeding (Liu et al., 2014), and balanced out by a decline in the soil profile not subject to tillage anymore. Furthermore, Liu et al. (2014) reported that the increase of SOC stocks is positively correlated with labile organic carbon under NT in northern China. In their study, microbial biomass was significantly increased under NT as well. Similarly, Q. Wang et al. (2008) also detected greater microbial biomass carbon and higher respiration rates per unit soil on the NT fields on the Loess Plateau. This implies that the newly formed SOC may have not been converted into stable organic matter. While other long-term benefits of NT have been well recognized, particularly in terms of increasing resistance to soil erosion, improving water use efficiency, and reducing labor and energy inputs (Pimentel et al., 1995; Leys et al., 2007; Tullberg et al., 2007; Knapen et al., 2008; Bai et al., 2009; He et al., 2010a; Li et al., 2011; Bhattacharyya et al., 2012), ignoring the liability/ stability of newly formed SOC questions the commonly used linear extrapolations on the overall storage of SOC in the soil profile and climate change mitigation attributed to conservation tillage (Li et al., 2012). 4.2. Contribution to ensure food security Unlike SOC stocks, the effect of NT on crop yields was positive in the Loess Plateau studies. The average crop yield increase for NT (+14.2%) observed in the plot studies (Fig. 5) is in strong contrast to the 5.7% decline found by Pittelkow et al. (2014) in their global assessment. More importantly, during years with annual
precipitation <500 mm, NT yields were 16.2% greater than CT yields on the Loess Plateau. Similar effects on yield increase from other conservation tillage practices on the Loess Plateau or arid northwest China have also been reported (He et al., 2010b; Bai et al., 2009; Wang et al., 2007; He et al., 2007a), as well on other crops, for instance, pea (Huang et al., 2008), maize (He et al., 2007b), millet and naked oats (He et al., 2010b). Such yield increase reduces the probability of food production falling below minimum thresholds of subsistence requirements, thereby increasing resilience to famine (Cavatassi et al., 2011). This is even more remarkable when considering that the area with marginal annual precipitation (<500 mm) for crop production on the Loess Plateau is likely to increase during the next 100 years by 31% (Fig. 7). Apart from the average and future yields, inter-annual variability also has to be considered. NT drought resilience on the Loess Plateau, illustrated by the greater yield improvements during years with below average precipitation, shows that long-term averages do not distinguish between sustainable intensification of agriculture and food security in dry years. However, the latter is equally relevant because famine is not caused by lack of global average production, but regional access to affordable food after poor harvests (Cavatassi et al., 2011). Increasing regional resilience to drought is therefore an essential NT benefit, especially for poor farmers. Finally, comparing yields and SOC stocks between plot treatments does not fully reflect the future feedback of NT on soil protection and productivity. For instance, on the Loess Plateau, up to 70% of arable land is affected by an annual soil loss of 20–25 t ha1 (Shi and Shao, 2000), far exceeding the threshold for sustainable use (e.g. 10 t ha1 proposed by Doetterl et al. (2012)). Such non-sustainable soil loss is not limited to the Loess Plateau, but affects 42% of cropland worldwide (Kuhn, 2014). If NT reduces erosion rates to avoid soil destruction (X. Wang et al., 2008), then even maintaining
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(2014). However, the positive effect was masked in their global assessment by an overall negative NT average on the 610 plots. The papers used by Pittelkow et al. (2014) rely mainly on data from the American East and Midwest, Europe, India and eastern China, out of which about 30% is wheat, 27% is maize, and the rest 43% from other types of crop. An overlay of their study locations and the areas classified as drylands by FAO (2008) show that 42% of their study sites are located in drylands (Appendix C). Yet, only approximately 30% of global croplands are situated in drylands (FAO, 2008), suggesting an over-representation of drylands by the plot studies in Pittelkow et al. (2014). This means that, as a result of global averaging, the negative results of NT from the underrepresented humid regions still outweighed the positive effects from the over-represented drylands. In addition, globally, 66% of wheat area and 82% of maize area is rainfed (Venkateswarlu and Shanker, 2012), demanding much more weight than the 30% wheat and 27% maize reflected in Pittelkow et al. (2014), if evaluating the potential effects of NT on global crop yields. The discrepancies between plot numbers and the extent of the areas they apply to, illustrate the risks that global averaging masks the benefits of NT in the context of a particular combination of eco-region and tillage practice (please also see our comment online following Pittelkow et al. (2014)). In particular for large regions such as the Loess Plateau, with croplands corresponding to 6.4% of all cropland in drylands (2.12 million km2 according to FAO (2008)), global averaging carries the risk of disregarding beneficial soil management practices such as NT in policy development aimed at achieving sustainable regional and national food production. 5. Conclusion
Fig. 7. The spatial distribution of mean annual precipitation in the Loess Plateau region in 2010, and the projected spatial distribution of mean annual precipitation in 2050 and 2100. The GIS data was adopted from Wan et al., (2014). The highlighted areas represent the areas, where no-tillage has shown the most pronounced advantage in increasing yield under current climate conditions. The six cities are the locations of the experimental sites from the reviewed literature listed in Appendix A.
yields and SOC stocks, without increasing the use of mineral fertilizer and the associated emissions, represent a significant contribution to sustainable food production. 4.3. Assessing NT benefits by agro-ecosystem The positive impact of NT on the Loess Plateau corresponds to the NT benefits in drylands acknowledged by Pittelkow et al.
This paper reviewed almost 20 years of NT plot experiments across the Chinese Loess Plateau, aiming to assess the long-term benefits of NT for mitigating and adapting to climate change, and thus contribute to the sustainable intensification of agriculture. Furthermore, the risks associated with global averaging of plot data were analyzed. The observed decreasing total SOC stocks changes with soil depth suggest that benefits of NT in sequestrating SOC are only concentrated to the immediate topsoil still subject to direct seeding. The total SOC stocks and the annual change in SOC stocks also declined over time, suggesting that the soil carbon sink reaches a maximum capacity after about 10 years of NT on the Loess Plateau. The temporal trend of SOC stocks observed in this study confirms some recent papers (Liu et al., 2014; Powlson et al., 2014), but contradicts the statement by Tang and Nan (2012) on the potential of NT on the Loess Plateau to mitigate climate change. As a consequence, the overall potential of NT in improving SOC stocks on the Loess Plateau is very likely to be over-estimated, if linearly extrapolating annual change derived from short-term observation to long-term estimation. The contrasting conclusions on NT crop yield derived from this study for the Chinese Loess Plateau, the FAO reports (FAO, 2011, 2008) and Pittelkow et al. (2014) illustrate that the contribution of NT to sustainable food production and food security is not yet fully understood. A key reason for the different results appears to be the global averaging of pairwise plot studies where all plots are given equal weighting, irrespective of the area they would apply to and the regional, national or international relevance of food production in this area. Instead of global averaging, the spatially weighted sum of impacts for each combination of eco-region and tillage practice, including future trends of climate change and soil quality on crop production and food security, should be considered when studying NT. Clearly, policy decisions should be based on agro-eco-region analysis, rather than global averaging, and should also include factors such as tillage system performance in dry years, especially the impact on future yields, soil loss and thus productivity
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decrease. The different interpretations of NT performance of global and regional meta-analysis also demonstrate that NT impacts are far from being adequately understood, but have to be seen as research in progress.
for sharing his GIS database, and the contribution of Samuel Kuonen to process the GIS database. Appendix A. See Table A1.
Acknowledgement The authors gratefully acknowledge Liangang Xiao for collating the data from published papers. We are grateful to Xiaoping Zhang
Appendix B. See Table B1.
Table A1 Information of the locations, cultivation years, crop yields, precipitation, water use efficiency of conventional tillage (CT) and no-tillage (NT) collected from the studies on the Chinese Loess Plateau investigated in this paper.
Chen et al. (2008)
He et al. (2009)
Huang et al. (2008)
Jin et al. (2007)
Jin et al. (2009)
Li et al. (2007)
Su et al. (2007)
Journal
Study location
Cultivation years
Crops
Australian Journal of Soil Research, 2008, 46, 645–651
Linfen, Shanxi (37 320 –38 60 N, 112 40 –113 260 E)
8
Wheat No
1999 422
Wheat No
2000 2001 2002 2003 2004 2005 2006 1998
Soil Use and Management, June Wuchuan, Inner Mongolia 2009, 25, 201–209 (38 6 N, 113 E)
Field Crops Research 107 (2008), Dingxi, Gansu (35 28 N, 43–55 104 44E)
Soil & Tillage Research 96 (2007), 131–144
Soil & Tillage Research 104 (2009), 115–120
Australian Journal of Soil Research, 2007, 45, 344–350
Luoyang, Henan (34 50 N, 113 E)
Luoyang, Henan (34 50 N, 113 E)
Linfeng, Shanxi (37 320 –38 60 N, 112 40 –113 260 E)
Agricultural Water Management Luoyang, Henan (34 50 N, 87 (2007), 307–314 113 E)
10
4
5
7
15
6
Irrigation Year
Annual precipitation (mm)
Average yield (t ha1)
Water use efficiency (t ha1 mm1)
CT
NT
CT
NT
3.79
3.27
17.2
15.1
328 443 418 447 669 308 424 159.1
1.46 2.91 3.52 3.64 4.12 1.91 3.5 1.071
2.48 3.08 3.68 3.51 4.01 2.71 4.43 1.181
11.8 9.5 10.9 9.7 10.1 10.3 NA 4.5
12.6 9.7 14 10.5 9.1 11.1 NA 4.9
Wheat No
1999 2000 2001 2002 2003 2004 2005 2006 2007 2002
190 274.9 268.9 254.9 167.2 309.7 286.4 206.1 192.6 351
1.142 1.365 1.234 1.321 1.057 1.486 1.289 1.116 1.104 2.1
1.324 1.42 1.348 1.478 1.326 1.511 1.511 1.426 1.365 2.3
4.3 4.2 3.8 4.2 4.4 4.2 3.6 4.1 4.2 7.1
5.1 4.3 4.2 4.8 5.4 4.2 4.1 5.2 4.9 7.8
Wheat No
2003 2004 2005 2001
565 345 453 775
1.5 2.2 2.6 3.922
2 2.3 3.1 4.107
4.7 9.1 8.8 0.53
6.1 8.5 9.7 0.54
Wheat No
2002 2003 2004 2005 2000
742 521 1182 724 573
5.169 4.019 4.347 4.801 4.218
4.897 4.778 4.27 5.292 3.856
0.71 0.7 0.38 0.67 NA
0.67 0.83 0.38 0.74 NA
Wheat No
2001 2002 2003 2004 2005 2006 1993
521 512 1038 707 586 NA 470
3.922 5.169 4.019 4.347 4.801 5.171 2.548
4.107 4.897 4.778 4.27 5.292 5.383 2.985
NA NA NA NA NA NA 9.4
NA NA NA NA NA NA 11.2
Wheat No
1994 1995 1996 1997 1998 1999 2000 2001 2002 2003 2004 2005 2006 2000
522 434 535 574 359 422 328 443 418 447 669 308 424 573
3.002 2.342 3.455 3.908 2.495 2.148 1.652 2.917 3 3.36 4.605 2.65 4.41 3.857
3.162 2.513 3.865 4.142 3.06 2.644 2.578 3.814 4.23 3.975 5.175 3.765 4.696 4.218
12.3 9.1 19.1 15.3 10.2 15.6 13.8 13.4 17.2 16.4 14.4 17.4 17.5 1.205
13 9.8 22.8 16.3 11.5 18.2 19.4 16.9 23.1 19.6 16.1 20.1 18.4 1.3
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163
Table A1 (Continued) Journal
Wang et al. Soil & Tillage Research 104 (2009) (2009), 192–197
Zhang et al. Soil & Tillage Research 102 (2009) (2009), 78–86
Study location
Linfeng, Shanxi (37 320 –38 60 N, 112 40 –113 260 E)
Heyang, Shanxi (35190 N, 110 40 E)
Cultivation years
6
3
Annual precipitation (mm)
Average yield (t ha1)
Water use efficiency (t ha1 mm1)
CT
NT
CT
NT
Wheat No
2001 2002 2003 2004 2005 1999
521 512 1038 707 586 422
4.021 4.469 3.541 4.741 4.118 3.79
4.607 5.007 4.355 5.225 4.662 3.27
1.112 1.106 1.106 1.004 1.17 17.2
1.137 1.201 1.201 1.162 1.291 15.1
Wheat No
2000 2001 2002 2003 2004 2005 2002
328 443 418 447 669 308 513.159
1.46 2.91 3.52 3.64 4.12 1.91 2.63
2.48 3.08 3.68 3.51 4.01 2.71 2.91
11.8 9.5 10.9 9.7 10.1 10.3 NA
12.6 9.7 14 10.5 9.1 11.1 NA
2.57 4.8
3.01 4.51
NA NA
NA NA
Crops
Irrigation Year
2003 435.3 2004 998.6
Table B1 Information of the locations, cultivation years, soil organic carbon stocks in different depths of conventional tillage (CT) and no-tillage (NT) collected from the studies on the Chinese Loess Plateau investigated in this paper. Journal
Location
Years Crop type
Irrigation Soil organic carbon stocks (t ha1) Soil depth (cm)
Chen et al. (2008)
Chen et al. (2009)
He et al. (2009)*
Jin et al. (2007)
Li et al. (2007)*
Liu et al. (2014)
Wang et al. (2008)
Australian Journal of Soil Research, 2008, 46, 645–651
Soil & Tillage Research 106 (2009), 85–94
New Zealand Journal of Crop and Horticultural Science, 2009, vol. 37: 157–166
Soil & Tillage Research 104 (2009), 115–120
Australian Journal of Soil Research, 2007, 45, 344–350
Geoderma 213 (2014), 379–384
Geoderma 144 (2008), 502–508
Linfen, Shanxi (37 320 –38 60 N, 112 40 –113 260 E)
Linfen, Shanxi (38 60 N, 1130 E)
11
16
Linfen, Shanxi (38 60 N, 113 E)
Luoyang, Henan (34 50 N, 113 E)
0
0
Linfeng, Shanxi (37 32 –38 6 N, 112 40 –113 260 E)
Linfeng, Shanxi (37 320 –38 60 N, 112 40 –113 260 E)
Linfen, Shanxi (38 60 N, 113 E)
8
5
15
17
16
Wheat
Wheat
Wheat
Wheat
Wheat
Wheat
Wheat
CT
No
0–10
No
10–20 20–30 total 0–15
7.24 7.98 3.51 4.81 20.65 26.03 19.6 22.2
No
15–30 Total 0–5
20.2 15.4 39.8 37.6 9.34 12.77
No
5–10 10–20 20–30 total 0–20
10.40 11.68 20.01 16.32 14.17 9.01 53.93 49.78 20.3 23.86
No
20–30 total 0–10
7.51 27.78 17.76
No
20–20 total 0–5
12.98 15.75 30.75 39.61 4.5 9.5
No
5–10 10–20 20–30 30–40 40–50 50–60 0–5
3.9 6.2 9.3 9.7 8.5 7.8 7.7 5.6 6.5 5.9 5.8 5.4 9.34 12.77
5–10 10–20 20–30 Total
10.40 11.68 20.01 16.32 14.17 9.01 53.93 49.78
NOTE: Soil organic carbon (SOC) stocks in the papers marked with * was calculated from soil organic matter content by multiplying a factor of 1.73.
9.91
NT 13.23
8.57 32.42 23.87
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Fig. C1. An overlay of the study locations in Pittelkow et al. (2014) and the areas classified as drylands by the FAO (2008) report. Green dots indicate the study locations inside drylands, whereas the red dots indicate the study locations outside drylands. Study locations, and aridity information were extracted from the supplement materials of Pittelkow et al. (2014).
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