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Soil carbon and nitrogen dynamics in a Vertisol following 50 years of no-tillage, crop stubble retention and nitrogen fertilization
Geoderma 358 (2020) 113996
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Soil carbon and nitrogen dynamics in a Vertisol following 50 years of notillage, crop stubble retention and nitrogen fertilization
T
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Pramod Jha1, , K.M. Hati1, Ram C. Dalal, Yash P. Dang, Peter M. Kopittke, Neal W. Menzies The University of Queensland, School of Agriculture and Food Sciences, St Lucia, 4072 Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Soil organic carbon Soil nitrogen Tillage Stubble management
Conservation agriculture commonly improves soil health and results in organic C sequestration. This study compared conventional tillage (CT) and no tillage (NT) systems, examining differences in stubble management [stubble burning (SB) and stubble retention (SR)] and N fertilization on soil C, C fractions and N. The experiment was established in 1968 on a Vertisol at Hermitage in Queensland, Australia. After 50 years of management; it was found that the tillage regime, the addition of N fertilizers, and the stubble management were all important in influencing soil C pools in 0–0.1 m of soil depth. Overall, the addition of N fertilizers and the tillage regime were the most important management factors. For example, the addition of N fertilizers at a rate of 90 kg ha−1 resulted in an average increase in SOC of 18%, while use of NT resulted in an average increase in SOC of 5%. Similarly, application of N fertilizers increased total soil N (TSN) concentrations by ≤25.5%, increased the stabilized SOC fraction by ≤26.6%, and increased microbial biomass C by ≤22%. Regardless, stubble management practices were also important, with the retention of stubble increasing TSN, increasing the stabilized C fraction, and increasing the microbial biomass C ≤ 30%. Finally, it was observed that the microbial metabolic quotient was 39% higher when stubble was burned and in the absence of N fertilization, indicating microbial stress under these treatments. The study clearly shows the importance of stubble retention and N fertilization in conservation agriculture for improvement in soil health and maintenance of soil C levels.
1. Introduction
management strategy that can increase SOC concentrations. Given that soils in these systems remain essentially undisturbed, the soil aggregates remain largely intact and they therefore provide greater physical protection to the entrapped C. The meta-analysis of West and Post (2002) showed that NT can increase SOC rapidly, especially at the soil surface. This is in agreement with other studies that have found that the increase in SOC is due to increased aggregation (Cambardella and Elliott, 1992; Six et al., 2000). Given that the increase in SOC observed in these systems is largely due to physical protection, maintaining the NT regime is important to ensuring that the SOC remains sequestered. Thus, soils managed with NT not only increase SOC, but they also have improved microbial functioning and availability of nutrients (Dalal et al., 1991; Thomas et al., 2007). There have been also contradictory reports on benefit of conservation tillage. Baker et al. (2007) opined that the effect of conservation tillage was found to sequester C, only up to 30 cm or less, although crop roots often extend much deeper. They further reported that in the few studies where sampling extended deeper than 30 cm, conservation tillage has shown no consistent accrual of SOC, instead showing a difference in the distribution of SOC, with
Soil organic matter (SOM) plays an important role in ecosystems by retaining and supplying plant nutrients, improving soil aggregation, reducing soil erosion, and enhancing water holding capacity (Tisdall and Oades, 1982; Brady and Weil, 2002). However, the use of soils for cropping results in the increased mineralization of soil organic C (SOC), depleting soil stocks by 30–60% compared to natural vegetation (Don et al., 2011; Poeplau et al., 2011; Kopittke et al., 2017). A range of soil management strategies can be used to increase SOC stocks in agricultural lands, with widespread implementation of such practices resulting in substantial sequestration of C. Currently, cropland accounts for 1600 million ha (12% of the total ice-free land) and permanent meadow and pasture account for ca. 3300 million ha (25% of ice-free land) (FAO, 2018). Thus, the development of new approaches to stabilize SOC is important in the use of soils as a C sink. Three factors influencing SOC levels are considered here, being tillage, stubble management, and the rate of N fertilization. No-till (NT) systems are the most widely studied agricultural
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Corresponding author. E-mail address: [email protected] (P. Jha). 1 Present address: Indian Institute of Soil Science, Nabi Bagh, Berasia Road, Bhopal, MP 462038, India. https://doi.org/10.1016/j.geoderma.2019.113996 Received 12 February 2019; Received in revised form 27 September 2019; Accepted 30 September 2019 0016-7061/ © 2019 Elsevier B.V. All rights reserved.
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biomass carbon) and N in soil. We hypothesized that 50 years of notillage, crop stubble retention and N fertilization will have substantial impacts on SOC pools and N dynamics in 0–0.1 m of soil depth.
higher concentrations near the surface in conservation tillage and higher concentrations in deeper layers under conventional tillage. Luo et al. (2010) after meta analysis of global data from 69 paired-experiments also reported that conversion from CT to NT significantly changed C distribution in the soil profile, but did not increase the total SOC except in double cropping systems. After adoption of NT, soil C increased in the surface 0.1 m of soil, but declined in the 0.2–0.4 m soil layer. Overall, adopting NT did not enhance soil total C stock in 0–0.4 m of soil depth. Similar conclusions were drawn by Chenu et al. (2019) while performing the meta analysis of NT data. Conventional tillage (CT) practices, including the repeated tillage of soil, can have adverse impacts on soil aggregation and the loss of SOC, leading to CO2 emissions and thereby increasing CO2 levels in the atmosphere (Baker et al., 2007; Sey et al., 2008). The retention of crop stubble in the field can also increase soil aggregation and the buildup of SOC, with the removal of stubble decreasing SOC levels (Mann et al., 2002; Kätterer and Andrén, 1999; Rasmussen and Parton, 1994). However, the extent to which SOC increases following the retention of crop stubble depends upon the quality and quantity of the stubble being retained (Chivenge et al., 2007). In a similar manner, the burning of crop stubble, which is commonly practiced in the tropics to control weeds and prepare fields for planting, decreases SOC and also causes the loss of nutrients and reduces soil fertility (Hemwong et al., 2008). Also, as the cultivation leads to decline in TOC, the more resistant charcoal fraction increases as a portion of the total C (Skjemstad et al., 2001). The final factor influencing SOC levels considered here is N fertilization. In a review of 137 sites, Alvarez (2005) concluded that N fertilizers increase SOC when crop residues are retained, although differences were observed between temperate and tropical regions. Neff et al. (2002) showed that N additions significantly accelerated the decomposition of the light soil C fractions (with decadal turnover times) while further stabilizing soil C compounds in the heavier, mineral-associated fractions (with multi-decadal to century lifetimes). Bhattacharyya et al. (2011) reported build up of total SOC, oxidizable soil organic C and its fractions with application of chemical (NPK) fertilizer. They further elaborated that application of farm yard manure (FYM) along with N resulted in larger proportion of total SOC in the recalcitrant pool than the treatments with mineral or no fertilizer, indicating that FYM application either promoted SOC stabilization or added stabilized organic C. While some authors have observed a decrease in soil C following long-term application of N fertilizer (Mulvaney et al., 2009), others have reported an increase in SOC with balanced inorganic fertilization (Bharadwaj and Omanwar, 1994; Schjonning et al., 1994; Hati et al., 2008). Paustian et al. (1997) described several mechanisms of SOC storage in response to crop stubble incorporation and N fertilization, but concluded that much unexplained variation exists between field experiments. Thus, the ultimate effects of management practices on SOC levels are complicated and remain unclear in many situations. In this regard, the quantitative analysis of the labile and recalcitrant fractions of SOC can be useful for obtaining a better understanding of C dynamics and their responses to tillage, stubble management, and fertilization. Indeed, the long-term adoption of a NT system in which stubble management is altered would have implications on SOC fractions and N dynamics. Effect of long term tillage, residue management and N fertilization impact on soil organic carbon was studied by the other authors also (Dalal et al., 2011; Sarker et al., 2018). However, their effect on different forms of carbon (like stabilized carbon, Charcoal C, carbon left after treatment with NaOCl) and labile C (as substrate induced respiration) were not studied earlier. We examined a long-term trial at Hermitage Research Station (Australia), which commenced in 1968, with this trial providing an excellent platform for monitoring the impact of long term tillage, stubble management and N fertilization impact on changes in different forms of C (charcoal C, stabilized C, C left after treatment with sodium hypochlorite, labile C in form of microbial
2. Materials and methods 2.1. Experimental site and treatments The field experiment was established at Hermitage Research Station (28°12′S, 152°06′E), Queensland, Australia, in December 1968. The site is situated in the sub-tropics, with a mean annual temperature of 17.5 °C, and mean monthly minimum and maximum temperatures of 2 and 17 °C in July and 15 and 30 °C in January. The site receives a mean annual rainfall of 685 mm, approximately 60% of which is received during the summer months from December to March. The soil is a black self-mulching cracking clay, being classified as a Vertisol (FAO, 1998), Vertosol (Australian Soil Classification, Isbell, 1996) or Ustic Pellustert (US Soil Taxonomy, Soil Survey Staff, 1999), developed on basaltic alluvium. It contains 65% clay, 24% silt, and 11% sand in the top 0.1 m depth. Soil pH (1:5 soil:water) for the 0.1 m layer varied from pH 6.9 for the treatment to which N fertilizer was applied to pH 7.6 for soils not receiving N fertilizer. The trial was in a factorial arrangement with two tillage practices (CT, NT), two crop stubble management practices [stubble burnt (SB) and stubble retained (SR)], and three N fertilization rates [no N fertilizer (N0), 30 kg N ha−1 y−1 (N30), and 90 kg N ha−1 y−1 (N90)]. The 12 treatments were arranged in 61.9 m × 6.4 m plots in a randomized block design with four replications, covering an area of 1.9 ha. Wheat (Triticum aestivum L) was grown in the experiment except for a short period of barley (Hordeum vulgare L. cv. Clipper) for 3 years (1975–1977) (Marley and Littler, 1989). The crop was sown and fertilized in the month of June and harvesting was done in the month of December. The treatments under CT generally involved three to four tillage operations with a chisel plough to approximately 0.1 m depth during the fallow period each year (December-June), while the treatments under NT were sprayed with herbicide to control weeds. The N90 treatments received urea at 46 kg N ha−1 y−1 during the first 8 years, 69 kg N ha−1 y−1 until 1996 and then at 90 kg N ha−1 y−1 thereafter. The fertilizer was applied at sowing at approximately 0.05 m depth in the middle of two rows. No crops were grown in 1982, 1991, 1994 and 2004 due to insufficient rainfall for sowing. Whilst the crop was being mechanically harvested, the crop stubble was retained in situ. Stubble was left on the soil surface in the NT treatment, but incorporated into the top 0.1 m depth by chisel plough in the CT treatment. In the SB treatment, crop residue was burned in situ immediately after harvest and before the first tillage operation (Marley and Littler, 1989). 2.2. Soil sampling and analysis Five soil samples from each plot were collected in May 2018 from a depth of 0–0.1 m. The five samples from each plot were bulked and sealed in plastic bags for transport to the laboratory. The samples were air-dried, ground to pass a 0.25 mm sieve for SOC and TSN analysis. Bulk density was measured in past several years and there was no significant effect of treatments on soil bulk density. Hence, stocks were computed from the past measured BD value, which was 1 Mg m−3 (Wang and Dalal, 2006; Dalal et al., 2011). Since the effect of treatment on soil BD was insignificant, there was no requirement to correct for equivalent mass basis. 2.3. Determination of SOC fractions Total organic C and TSN were determined by high-temperature (1200 °C) oxidative combustion followed by non-dispersive infrared detection of CO2 using LECO CN analyzer (LECO Corporation, St. Joseph, MI, Model CN 928 series). Recalcitrant organic C (NaOCl 2
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Fig.1. Impacts of tillage, residue management and N fertilization on (A) soil organic C (SOC), (B) total soil N (TSN) stocks and (C) C:N ratio of the soil. The vertical bars are the standard errors of the mean.
PP system). The MBC was calculated as (Horwath and Paul, 1994):
oxidizable C) was determined by oxidation with NaOCl at room temperature following a modified method of Kaiser et al. (2002). The soil (1.0 g) was oxidized with 50 ml of 6% (wt/wt) NaOCl that had been adjusted to pH 8 with concentrated HCl, at 25 °C for 18 h. The residue was centrifuged at 1000g for 20 min, decanted, and washed thoroughly with deionized water. This process was repeated a total of three times. The C and N concentrations of all dried fractions (40 °C) were measured following chemical treatment using the LECO CN analyzer. The charcoal C concentration of the soil was determined using the method outlined by Kurth et al. (2006). Briefly, 1.0 g sample of dry soil was weighed into a 250 ml Erlenmeyer flask and treated with 20 ml of 30% H2O2 and 10 ml of 1 M HNO3. The flasks were occasionally swirled at room temperature over a 30 min period and then heated to 100 °C on a hotplate for 16 h. Samples were swirled occasionally and observed for the occurrence of effervescence. If a sample was still active, the flask was returned to the heating plate for an additional 4 h and occasionally swirled. When the digestion was complete (no further effervescence), samples were filtered using Whatman No. 2 filter paper, dried and homogenized with a mortar and pestle, and the total C concentration determined by dry combustion. The total C measured after digestion is reported as total charcoal C assuming that all non-charcoal C was consumed in the digestion process (Kurth et al., 2006).
MBC = 40.04y + 0.37 where MBC is the microbial biomass C (mg kg−1) and y is the rate of CO2 evolution (mg C kg−1 soil).
2.5. Metabolic quotient We determined the microbial metabolic quotient for CO2 (qCO2, h−1), or specific respiration rate of the biomass, which represents the CO2-C produced (mg C kg−1 soil ha−1) per unit of MBC (mg C kg−1 soil) over time using the laboratory incubation method. The rate of CO2 production from the soil samples after a 10-d incubation period was attributed to basal respiration. For this, a C-mineralization experiment was conducted for 10 d. The moisture content of 50 g air-dry soil was adjusted to 60% of water-holding capacity prior to its incubation in an air-tight plastic jar at a constant temperature of 25 °C. The microbial metabolic quotient (qCO2) (h−1) was calculated by dividing basal respiration by MBC.
2.6. Mineral N concentration 2.4. Soil microbial biomass carbon A total of 3 g of air dried soil was mixed with 30 ml of 2 M KCl and shaken for 1 h on a rotary shaker. The samples were then centrifuged at 1000g and subsamples were drawn using Swinnex filter holder fitted with glass microfiber filter for determination of nitrate-N by the automated cadmium reduction method (Baird and Bridgewater, 2017) using segmented flow analyzer (SEAL analytical, Norderstedt, Germany).
Microbial biomass C (MBC) was measured using the substrate-induced respiration (SIR) method (Horwath and Paul, 1994), in which 200 mg of glucose was dissolved in deionized water and added to 25 g dry soil to bring it to 60% of its water-holding capacity. The soil was incubated in plastic jars for 6 h at 25 °C, and the amount of CO2 that accumulated in the head space was measured by CO2 analyzer (WMA-5, 3
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30% of total SOC under the different treatments. It was found that 50 years of stubble retention and N fertilization significantly affected the stabilized C concentration in soil (Fig. 3A). Specifically, it was found that the management of the stubble and the N fertilization on the stabilized C concentration was the opposite to their effects on the charcoal C concentration. For example, the retention of stubble increased the stabilized C concentration by 18.6% compared to the SB treatments. Similarly, application of N fertilizer at a rate of 30 kg ha−1 increased the stabilized C by 18%, with 90 kg ha−1 increasing it by 26%. Furthermore, the effect of SR on C stabilization was more pronounced when it was coupled with NT. In contrast to stabilized C, no significant differences were observed for stabilized N between the various treatments (Fig. 3B). Regarding the C:N ratio of this fraction, it was observed that retention of stubble resulted in a significantly higher C:N ratio (13.5) compared to the SB treatments (11.4) (Fig. 3C).
2.7. Statistical analysis Data were statistically analyzed using three-way analysis of variance (ANOVA) procedure of SPSS version 25. The ANOVA and leastsignificant-difference (LSD) were calculated to compare variable means under different treatments. 3. Results 3.1. Soil total C, N stocks and C:N ratio After 50 years of management, the SOC stock in the top 0.1 m of the soil profile was significantly affected by both tillage and N fertilization. Specifically, the SOC stocks as average of N fertilization rates increased 19.9 Mg ha−1 in CT to 20.9 Mg ha−1 in NT, while SOC as average of tillage system increased from 18.5 Mg ha−1 in N0 to 20.8 Mg ha−1 in N30 and to 21.9 Mg ha−1 in N90, with the increase at N90 representing a 18% increase (Fig. 1A). In contrast, residue management did not significantly affect the SOC stocks, with the average SOC in the SR treatment (20.5 Mg ha−1) being similar to that in the SB treatment (20.3 Mg ha−1) (Fig. 1A). Overall, the treatment with the highest SOC stock was NT-SR-N90 (22.9 Mg ha−1), which was statistically at par with NT-SR-N30 and NT-SB-N90 and was in contrast to CT-SB-N0 and CT-SR-N0 which had the lowest SOC stocks (18.2 Mg ha−1). Thus, the greatest increases in SOC occurred at higher rates of N fertilization, with the use of NT in the absence of fertilization resulting in a more modest increase in SOC. Indeed, after 50 years, the use of NT together with the application of 90 kg N ha−1 and stubble retention resulted in a 22.2% increase in SOC stock when compared to the CT-SB-N0 treatment. Not only did management practices alter SOC, but also TSN. Again, the largest increase in TSN stock was observed upon the application of N fertilizers, with TSN increasing from an average of 1.05 at N0 to 1.32 Mg ha−1 at N90, with this representing a 25.7% increase (Fig. 1B). In addition, the retention of stubble also significantly (P < 0.001) increased the TSN stock in the soil, from an average of 1.14–1.26 Mg ha−1 (Fig. 1B). Also, the tillage regime was found to influence TSN stock, being 1.18 Mg ha−1 in the CT and 1.23 Mg ha−1 under NT treatments. For the C:N ratio, tillage practices alone did not cause significant differences. However, both stubble management and N fertilization significantly affected the C:N ratio of the soil. Specifically, the average C:N ratio changed from 16.3 for SR to 17.8 for SB (Fig. 1C), which was associated with the increase in TSN in soils where stubble was retained. In a similar manner, the average C:N ratio changed from 17.6 for N0 to 16.7 for N90 (Fig. 1C).
3.4. Soil microbial biomass carbon (MBC) After 50 years of management, it was found that MBC was significantly affected by tillage practices, stubble management and N fertilization. There was a positive and significant interaction between the tillage and stubble management; combinations of SR with CT, and N fertilization with CT, all resulting in higher MBC. It is interesting to note that the use of CT increased MBC (286 mg kg−1) compared to NT (243 mg kg−1), especially in the SR treatments (Fig. 4). Not only MBC was influenced by tillage, but also by stubble retention. It was found that the retention of stubble resulted in 30% higher MBC than in soils with stubble burned treatment. Similarly, N fertilization had a significant and positive effect on MBC values increasing from 234 mg kg−1 in N0 treatment to 274 mg kg−1 at 30 N ha−1 and 285 mg kg−1 at 90 kg N ha−1 (Fig. 4). 3.5. Metabolic quotient In the present study, we found a significant interaction between stubble management and N fertilizer application; however, 50 years of NT practice did not affect the metabolic quotient of the soil (Fig. 5). It was found that the metabolic quotient was significantly higher (2.99 × 10−3 h−1) in the SB treatments compared to that in the SR treatments (2.15 × 10−3 h−1) (Fig. 5A). The comparatively high additions (but low losses) of labile C in the SR treatment also means that more C is sequestered in the soil in stubble retained system; thus, this system contributes less to atmospheric CO2 than stubble burned system. We observed higher SOM accumulation under the SR than SB treatments. Furthermore, N fertilization significantly reduced the metabolic quotient of the soil, being significantly higher in the N0 treatment (3.80 × 10−3 h−1) than the N90 treatments where it was ca. 50% lower (1.7 × 10−3 h−1) (Fig. 5B). Indeed, even the application of 30 kg of N ha−1 significantly reduced the metabolic quotient (2.13 × 10−3 h−1). Interaction of N fertilization and stubble management was also significantly affected the metabolic quotient of the soil (Fig. 5C).
3.2. Charcoal C concentration (carbon remaining after treatment with H2O2 and HNO3) Overall, the charcoal C concentration ranged from 12 to 22% of SOC under the various treatments. However, the charcoal concentration of soil was found to be significantly affected by the stubble management practices, but not by either tillage or N fertilization (Fig. 2A). The burning of the stubble tended to increase the charcoal C concentration, with the highest value being 4.8 g kg−1 for the SB treatment with NT. Indeed, the charcoal C concentration was generally 3.3–4.8 g kg−1 in the SB soils but 2.3–3.3 g kg−1in the SR soils, being an average of 35.7% higher. No significant difference in charcoal total N concentration was recorded under different treatments (Fig. 2B). Finally, burning of stubble resulted in significantly (P = 0.002) higher charcoal C:N ratio (6.7) relative to the SR treatments (5.1) (Fig. 2C).
3.6. Nitrate-N concentration It was observed that the NO3−-N concentration of the soil that was collected after the harvest of wheat crop was affected by both stubble management and N fertilization. Tillage and N interaction effect on soil nitrate-N concentration was also found significant. It is interesting to note that nitrate-N concentration of SB treatment (19.8 mg kg−1) was significantly higher than the SR treatment (14.5 mg kg−1) (Fig. 6B). Application of N resulted in significantly higher NO3−-N concentration in soil, producing increases of 77% at 30 kg ha−1 and 233% at 90 kg ha−1 as compared to N0 treatment (Fig. 6A). Nitrogen application along with the CT practice maintained higher NO3−-N concentration in the soil in comparison to NT practice (Fig. 6C).
3.3. Soil carbon remaining after treatment with sodium hypochlorite (stabilized C) In the present study, we found that the stable C ranged from 18 to 4
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Fig. 2. Impacts of tillage, residue management and N fertilization on (A) charcoal C, (B) charcoal N and (C) C:N ratio. The vertical bars are the standard errors of the mean.
4. Discussions
reported that use of NT/SR and N fertilizer had not increased SOC stocks over time; although SR reduced loss of carbon in the 0–0.1 and 0–0.3 m layers and there was a trend towards reduced loss of carbon under NT in the 0–0.1 m. In a similar study at Central Ohio (USA), Duiker and Lal (1999) and Jha et al. (2017) reported a linear increase in SOC concentration with increasing rates of residue applied. Several authors have reported increase in SOC stock with balanced inorganic fertilization (Bharadwaj and Omanwar, 1994; Jha et al., 2014). Similarly, the largest increase in TSN was recorded with the highest rate of N application in our study (Fig. 1). Similar results were reported by Zhang et al. (2016) who examined the long-term impact of NT and stubble retention on soil N fractions after 22 years on a Chromic Cambisol. These authors also found that the use of no-till coupled with stubble retention was an effective management method for improving soil N stocks and increasing soil fertility. Similar findings have also been reported by others, with NT increasing the TSN compared to CT (Varvel and Wilhelm, 2011; Chen et al., 2009). Stubble burning significantly increased the charcoal C content of the soil (Fig. 2). This higher charcoal C concentration observed in the SB treatments was presumably due to the formation of char material due to long-term practice of stubble burning. During stubble burning, a fraction of stubble and SOM is converted into charcoal, a relatively stable C form. High levels of charcoal C resulting from repeated historical burning of grasslands, open woodlands, and agricultural crop stubbles have also been reported in soils from Australia (Skjemstad et al., 2002). Charcoal C is abundant in dark‐colored soils, affected by frequent vegetation burning, thus likely contributing to the highly stable aromatic components of SOM (Schmidt and Noack, 2000). Similar results have also been reported by Skjemstad et al. (2002), who reported that charcoal C constitutes 35% of SOC in US soils, while Lehmann et al. (2008) reported an average charcoal concentration of 33% of SOC in
Analysis of soil samples collected in 2018 from Hermitage site indicated that the use of NT, SR, and N fertilization had increased SOC stocks in the top 0.1 m of the soil compared to CT, SB, and no nitrogen fertilizer application (Fig. 1). Wang and Dalal (2006) also reported treatment effects in the 0–0.1 m depth were interactive and maximum SOC sequestration was achieved under the NT with SR and N fertilization. Our findings are in general agreement with those of Dalal et al. (1991) who studied the same site at Hermitage but after only 20 years of NT. These authors reported that residue retention increased organic C at 0–0.025, 0.025–0.05 and 0.05–0.1 m depths in soil under NT. Effects of fertilizer N application on organic C were primarily confined to 0–0.025 m depth although significant differences could be measured also in the 0–0.05 m layer. Dalal et al. (2011) further reported that after 40 years of treatment imposition, crop stubble and N fertilizer interactively increased SOC stocks at 0–0.10 m depth. They reported that fertilizer N application had no significant effect on SOC stocks when crop stubble was not retained. When crop stubble was retained, increasing rate of N fertilizer application resulted in higher SOC stocks. We also observed slightly higher SOC stock in SR than SB after 50 years of cropping, although the difference was insignificant. Our present findings are also in accordance with Mazzoncinia et al. (2016), for example, who reported a 22% increase in SOC (and TSN) content to a depth of 0.3 m in continuous NT treatments after 28 years on a Typic Xerofluvent. In the present study, the positive effect of N fertilization on SOC stocks was due to an increase in net primary productivity (Alvarez 2005; Christopher and Lal 2007; Ramirez et al.,2012). Yield response to N application (90 kg N ha−1 y−1) ranged from 9.50 to 24.9% under different treatments over N omission treatment (Dalal et al., 2011). Page et al. (2013) while analyzing time series data of Hermitage trial 5
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Fig. 3. Impacts of tillage, residue management and N fertilization on (A) stabilized C, (B) N concentrations and (C) C:N ratio. The vertical bars are the standard errors of the mean.
oxidizable C giving a good indication of the SOC fraction that has been stabilized over a period of time. Zimmermann et al. (2007) reported that isolation of C fraction after oxidation with NaOCl is a more suitable procedure for obtaining an operationally-defined stable organic matter fraction from soil. MBC was significantly affected by tillage practices, stubble management and N fertilization (Fig. 4). In the present study, CT had significantly higher MBC as compared to NT. This is in agreement with previous observations that tillage exposes SOM and increases mineralization, with mineralization of SOM occurring more rapidly in the CT
Australian soils. Long term practice of stubble retention and N fertilization significantly increased stabilized C in soil (Fig. 3). Collins et al. (2000) also reported that no-till increased resistant C by 20% in the surface 0.2 m at the three different sites in the US Corn Belt derived from both forest and prairie vegetation. The use of NaOCl has been proposed to remove less protected SOC to obtain a chemically resistant (stable) SOC fraction (Kleber et al., 2005; Mikutta et al., 2005). Indeed, such oxidative treatment may mimic biodegradation of the less protected SOC (Plante et al., 2004; Mikutta et al., 2006), with measurement of NaOCl
Fig. 4. Impacts of tillage, residue management and N fertilization on microbial biomass C (MBC) of the soil. The vertical bars are the standard errors of the mean. 6
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Fig. 5. Impacts of tillage, residue management and N fertilization on metabolic quotient (MQ) of the soil. In (A), values are the mean across N and tillage practices. In (B), values are the mean across residue management and tillage practices. In (C), values are the mean across all tillage practices. The vertical bars are the standard errors of the mean.
significantly affected by both stubble management and N fertilization (Fig. 6). Burning of stubble resulted in higher NO3−-N concentration in soil in comparison to SR treatment. Heat generated during fire induces the chemical oxidation of SOM, thereby altering C and N transformations. Choromanska and DeLuca (2002), while working on forest mineral soil which was exposed to fire, also reported lower basal respiration rates and lower concentrations of MBC, potentially mineralizable N, soluble hexose sugars, and NH4+-N, but higher NO3−N concentrations than in soil not exposed to fire. DeLuca et al. (2006) also reported charcoal produced due to forest fire significantly increased the nitrification potential, net nitrification, gross nitrification, but decreased the solution concentrations of plant secondary compounds (phenolics). Higher NO3−-N in fertilized treatments was due to conversion of added NH4+-N to NO3−-N. Higher NO3−-N concentration under the CT practice was probably due to higher organic matter mineralization under the CT as compared to NT practice, as shown by the higher microbial quotient in the former. After 50 years of management; N fertilization significantly affected SOC and TSN stocks. The retention of stubble increased TSN and stabilized C fraction, and increased the microbial biomass carbon in soil. The microbial metabolic quotient was significantly higher when stubble was burned and in the absence of N fertilization. Burning of stubbles significantly increased charcoal C concentration in soil.
than the NT treatments (Sarker et al., 2018). Sarker et al. (2018) also recorded higher MBC and MBN in the CT treatments in comparison to NT treatments. Moreover, differences in the availability of labile C and mineral N, in particular nitrate-N into the systems, may have also stimulated the growth of soil microbial population to enhance SOM decomposition (Chen et al., 2014). These results agree with findings from other long-term tillage studies, that soil disturbance increased the decomposition of SOM in surface layers due to higher microbial biomass and activity (Dalal and Chan, 2001), which in turn increased nutrient cycling (Bimüller et al., 2016). Geisseler and Scow (2014), while analyzing datasets from 64 long-term trials from around the world, also reported that mineral fertilizer application led to a 15.1% increase in the MBC as compared to unfertilized control treatments. They also noted that mineral fertilization increased SOC concentration, a factor contributing to the increase in MBC with mineral fertilization. The microbial metabolic quotient (respiration-to-biomass ratio), or qCO2, conceptually based on Odum's theory of ecosystem succession, is widely used as an index of ecosystem development and disturbance. Compared to SOC itself, qCO2 is likely more responsive to short term soil changes. A lower qCO2 is indicative of improved soil biophysical conditions resulting from amended soil with organic matter (Powlson and Jenkinson, 1981), but a higher qCO2 is indicative of soil degradation under intensive land use (Masciandaro et al., 1998). Stubble retention and N fertilization had significantly lowered microbial metabolic quotient of soil in comparison to SB and no N application (Fig. 5). These findings give an indication that the N application lowers C turnover while promoting microbial health in soils in the long term. These effects were probably due to improved microbial habitats and alleviated environmental stresses. Nitrate N concentration in soil after the harvest of wheat was
5. Conclusions After 50 years of management, application of N fertilizers were found to significantly increase SOC stocks in surface 0.1 m of the soil profile. The application of nitrogen fertilizers also increased total soil nitrogen as did stubble retention, but tillage did not influence the total 7
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Fig. 6. Impacts of tillage, residue management and N fertilization on nitrate-N concentration in the soil. In (A), values are the mean across all tillage and residue management practices. In (B), values are the mean across all N and tillage practices. In (C), values are the mean across all residue management treatments. The vertical bars are the standard errors of the mean.
References
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements Pramod Jha thanks the Department of Education, Australian Government for financial support received through Endeavour Research Fellowship (Recipient ID No. 6510-2018).
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.geoderma.2019.113996. 8
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Soil carbon and nitrogen dynamics in a Vertisol following 50 years of notillage, crop stubble retention and nitrogen fertilization
T
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Pramod Jha1, , K.M. Hati1, Ram C. Dalal, Yash P. Dang, Peter M. Kopittke, Neal W. Menzies The University of Queensland, School of Agriculture and Food Sciences, St Lucia, 4072 Australia
A R T I C LE I N FO
A B S T R A C T
Keywords: Soil organic carbon Soil nitrogen Tillage Stubble management
Conservation agriculture commonly improves soil health and results in organic C sequestration. This study compared conventional tillage (CT) and no tillage (NT) systems, examining differences in stubble management [stubble burning (SB) and stubble retention (SR)] and N fertilization on soil C, C fractions and N. The experiment was established in 1968 on a Vertisol at Hermitage in Queensland, Australia. After 50 years of management; it was found that the tillage regime, the addition of N fertilizers, and the stubble management were all important in influencing soil C pools in 0–0.1 m of soil depth. Overall, the addition of N fertilizers and the tillage regime were the most important management factors. For example, the addition of N fertilizers at a rate of 90 kg ha−1 resulted in an average increase in SOC of 18%, while use of NT resulted in an average increase in SOC of 5%. Similarly, application of N fertilizers increased total soil N (TSN) concentrations by ≤25.5%, increased the stabilized SOC fraction by ≤26.6%, and increased microbial biomass C by ≤22%. Regardless, stubble management practices were also important, with the retention of stubble increasing TSN, increasing the stabilized C fraction, and increasing the microbial biomass C ≤ 30%. Finally, it was observed that the microbial metabolic quotient was 39% higher when stubble was burned and in the absence of N fertilization, indicating microbial stress under these treatments. The study clearly shows the importance of stubble retention and N fertilization in conservation agriculture for improvement in soil health and maintenance of soil C levels.
1. Introduction
management strategy that can increase SOC concentrations. Given that soils in these systems remain essentially undisturbed, the soil aggregates remain largely intact and they therefore provide greater physical protection to the entrapped C. The meta-analysis of West and Post (2002) showed that NT can increase SOC rapidly, especially at the soil surface. This is in agreement with other studies that have found that the increase in SOC is due to increased aggregation (Cambardella and Elliott, 1992; Six et al., 2000). Given that the increase in SOC observed in these systems is largely due to physical protection, maintaining the NT regime is important to ensuring that the SOC remains sequestered. Thus, soils managed with NT not only increase SOC, but they also have improved microbial functioning and availability of nutrients (Dalal et al., 1991; Thomas et al., 2007). There have been also contradictory reports on benefit of conservation tillage. Baker et al. (2007) opined that the effect of conservation tillage was found to sequester C, only up to 30 cm or less, although crop roots often extend much deeper. They further reported that in the few studies where sampling extended deeper than 30 cm, conservation tillage has shown no consistent accrual of SOC, instead showing a difference in the distribution of SOC, with
Soil organic matter (SOM) plays an important role in ecosystems by retaining and supplying plant nutrients, improving soil aggregation, reducing soil erosion, and enhancing water holding capacity (Tisdall and Oades, 1982; Brady and Weil, 2002). However, the use of soils for cropping results in the increased mineralization of soil organic C (SOC), depleting soil stocks by 30–60% compared to natural vegetation (Don et al., 2011; Poeplau et al., 2011; Kopittke et al., 2017). A range of soil management strategies can be used to increase SOC stocks in agricultural lands, with widespread implementation of such practices resulting in substantial sequestration of C. Currently, cropland accounts for 1600 million ha (12% of the total ice-free land) and permanent meadow and pasture account for ca. 3300 million ha (25% of ice-free land) (FAO, 2018). Thus, the development of new approaches to stabilize SOC is important in the use of soils as a C sink. Three factors influencing SOC levels are considered here, being tillage, stubble management, and the rate of N fertilization. No-till (NT) systems are the most widely studied agricultural
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Corresponding author. E-mail address: [email protected] (P. Jha). 1 Present address: Indian Institute of Soil Science, Nabi Bagh, Berasia Road, Bhopal, MP 462038, India. https://doi.org/10.1016/j.geoderma.2019.113996 Received 12 February 2019; Received in revised form 27 September 2019; Accepted 30 September 2019 0016-7061/ © 2019 Elsevier B.V. All rights reserved.
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biomass carbon) and N in soil. We hypothesized that 50 years of notillage, crop stubble retention and N fertilization will have substantial impacts on SOC pools and N dynamics in 0–0.1 m of soil depth.
higher concentrations near the surface in conservation tillage and higher concentrations in deeper layers under conventional tillage. Luo et al. (2010) after meta analysis of global data from 69 paired-experiments also reported that conversion from CT to NT significantly changed C distribution in the soil profile, but did not increase the total SOC except in double cropping systems. After adoption of NT, soil C increased in the surface 0.1 m of soil, but declined in the 0.2–0.4 m soil layer. Overall, adopting NT did not enhance soil total C stock in 0–0.4 m of soil depth. Similar conclusions were drawn by Chenu et al. (2019) while performing the meta analysis of NT data. Conventional tillage (CT) practices, including the repeated tillage of soil, can have adverse impacts on soil aggregation and the loss of SOC, leading to CO2 emissions and thereby increasing CO2 levels in the atmosphere (Baker et al., 2007; Sey et al., 2008). The retention of crop stubble in the field can also increase soil aggregation and the buildup of SOC, with the removal of stubble decreasing SOC levels (Mann et al., 2002; Kätterer and Andrén, 1999; Rasmussen and Parton, 1994). However, the extent to which SOC increases following the retention of crop stubble depends upon the quality and quantity of the stubble being retained (Chivenge et al., 2007). In a similar manner, the burning of crop stubble, which is commonly practiced in the tropics to control weeds and prepare fields for planting, decreases SOC and also causes the loss of nutrients and reduces soil fertility (Hemwong et al., 2008). Also, as the cultivation leads to decline in TOC, the more resistant charcoal fraction increases as a portion of the total C (Skjemstad et al., 2001). The final factor influencing SOC levels considered here is N fertilization. In a review of 137 sites, Alvarez (2005) concluded that N fertilizers increase SOC when crop residues are retained, although differences were observed between temperate and tropical regions. Neff et al. (2002) showed that N additions significantly accelerated the decomposition of the light soil C fractions (with decadal turnover times) while further stabilizing soil C compounds in the heavier, mineral-associated fractions (with multi-decadal to century lifetimes). Bhattacharyya et al. (2011) reported build up of total SOC, oxidizable soil organic C and its fractions with application of chemical (NPK) fertilizer. They further elaborated that application of farm yard manure (FYM) along with N resulted in larger proportion of total SOC in the recalcitrant pool than the treatments with mineral or no fertilizer, indicating that FYM application either promoted SOC stabilization or added stabilized organic C. While some authors have observed a decrease in soil C following long-term application of N fertilizer (Mulvaney et al., 2009), others have reported an increase in SOC with balanced inorganic fertilization (Bharadwaj and Omanwar, 1994; Schjonning et al., 1994; Hati et al., 2008). Paustian et al. (1997) described several mechanisms of SOC storage in response to crop stubble incorporation and N fertilization, but concluded that much unexplained variation exists between field experiments. Thus, the ultimate effects of management practices on SOC levels are complicated and remain unclear in many situations. In this regard, the quantitative analysis of the labile and recalcitrant fractions of SOC can be useful for obtaining a better understanding of C dynamics and their responses to tillage, stubble management, and fertilization. Indeed, the long-term adoption of a NT system in which stubble management is altered would have implications on SOC fractions and N dynamics. Effect of long term tillage, residue management and N fertilization impact on soil organic carbon was studied by the other authors also (Dalal et al., 2011; Sarker et al., 2018). However, their effect on different forms of carbon (like stabilized carbon, Charcoal C, carbon left after treatment with NaOCl) and labile C (as substrate induced respiration) were not studied earlier. We examined a long-term trial at Hermitage Research Station (Australia), which commenced in 1968, with this trial providing an excellent platform for monitoring the impact of long term tillage, stubble management and N fertilization impact on changes in different forms of C (charcoal C, stabilized C, C left after treatment with sodium hypochlorite, labile C in form of microbial
2. Materials and methods 2.1. Experimental site and treatments The field experiment was established at Hermitage Research Station (28°12′S, 152°06′E), Queensland, Australia, in December 1968. The site is situated in the sub-tropics, with a mean annual temperature of 17.5 °C, and mean monthly minimum and maximum temperatures of 2 and 17 °C in July and 15 and 30 °C in January. The site receives a mean annual rainfall of 685 mm, approximately 60% of which is received during the summer months from December to March. The soil is a black self-mulching cracking clay, being classified as a Vertisol (FAO, 1998), Vertosol (Australian Soil Classification, Isbell, 1996) or Ustic Pellustert (US Soil Taxonomy, Soil Survey Staff, 1999), developed on basaltic alluvium. It contains 65% clay, 24% silt, and 11% sand in the top 0.1 m depth. Soil pH (1:5 soil:water) for the 0.1 m layer varied from pH 6.9 for the treatment to which N fertilizer was applied to pH 7.6 for soils not receiving N fertilizer. The trial was in a factorial arrangement with two tillage practices (CT, NT), two crop stubble management practices [stubble burnt (SB) and stubble retained (SR)], and three N fertilization rates [no N fertilizer (N0), 30 kg N ha−1 y−1 (N30), and 90 kg N ha−1 y−1 (N90)]. The 12 treatments were arranged in 61.9 m × 6.4 m plots in a randomized block design with four replications, covering an area of 1.9 ha. Wheat (Triticum aestivum L) was grown in the experiment except for a short period of barley (Hordeum vulgare L. cv. Clipper) for 3 years (1975–1977) (Marley and Littler, 1989). The crop was sown and fertilized in the month of June and harvesting was done in the month of December. The treatments under CT generally involved three to four tillage operations with a chisel plough to approximately 0.1 m depth during the fallow period each year (December-June), while the treatments under NT were sprayed with herbicide to control weeds. The N90 treatments received urea at 46 kg N ha−1 y−1 during the first 8 years, 69 kg N ha−1 y−1 until 1996 and then at 90 kg N ha−1 y−1 thereafter. The fertilizer was applied at sowing at approximately 0.05 m depth in the middle of two rows. No crops were grown in 1982, 1991, 1994 and 2004 due to insufficient rainfall for sowing. Whilst the crop was being mechanically harvested, the crop stubble was retained in situ. Stubble was left on the soil surface in the NT treatment, but incorporated into the top 0.1 m depth by chisel plough in the CT treatment. In the SB treatment, crop residue was burned in situ immediately after harvest and before the first tillage operation (Marley and Littler, 1989). 2.2. Soil sampling and analysis Five soil samples from each plot were collected in May 2018 from a depth of 0–0.1 m. The five samples from each plot were bulked and sealed in plastic bags for transport to the laboratory. The samples were air-dried, ground to pass a 0.25 mm sieve for SOC and TSN analysis. Bulk density was measured in past several years and there was no significant effect of treatments on soil bulk density. Hence, stocks were computed from the past measured BD value, which was 1 Mg m−3 (Wang and Dalal, 2006; Dalal et al., 2011). Since the effect of treatment on soil BD was insignificant, there was no requirement to correct for equivalent mass basis. 2.3. Determination of SOC fractions Total organic C and TSN were determined by high-temperature (1200 °C) oxidative combustion followed by non-dispersive infrared detection of CO2 using LECO CN analyzer (LECO Corporation, St. Joseph, MI, Model CN 928 series). Recalcitrant organic C (NaOCl 2
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Fig.1. Impacts of tillage, residue management and N fertilization on (A) soil organic C (SOC), (B) total soil N (TSN) stocks and (C) C:N ratio of the soil. The vertical bars are the standard errors of the mean.
PP system). The MBC was calculated as (Horwath and Paul, 1994):
oxidizable C) was determined by oxidation with NaOCl at room temperature following a modified method of Kaiser et al. (2002). The soil (1.0 g) was oxidized with 50 ml of 6% (wt/wt) NaOCl that had been adjusted to pH 8 with concentrated HCl, at 25 °C for 18 h. The residue was centrifuged at 1000g for 20 min, decanted, and washed thoroughly with deionized water. This process was repeated a total of three times. The C and N concentrations of all dried fractions (40 °C) were measured following chemical treatment using the LECO CN analyzer. The charcoal C concentration of the soil was determined using the method outlined by Kurth et al. (2006). Briefly, 1.0 g sample of dry soil was weighed into a 250 ml Erlenmeyer flask and treated with 20 ml of 30% H2O2 and 10 ml of 1 M HNO3. The flasks were occasionally swirled at room temperature over a 30 min period and then heated to 100 °C on a hotplate for 16 h. Samples were swirled occasionally and observed for the occurrence of effervescence. If a sample was still active, the flask was returned to the heating plate for an additional 4 h and occasionally swirled. When the digestion was complete (no further effervescence), samples were filtered using Whatman No. 2 filter paper, dried and homogenized with a mortar and pestle, and the total C concentration determined by dry combustion. The total C measured after digestion is reported as total charcoal C assuming that all non-charcoal C was consumed in the digestion process (Kurth et al., 2006).
MBC = 40.04y + 0.37 where MBC is the microbial biomass C (mg kg−1) and y is the rate of CO2 evolution (mg C kg−1 soil).
2.5. Metabolic quotient We determined the microbial metabolic quotient for CO2 (qCO2, h−1), or specific respiration rate of the biomass, which represents the CO2-C produced (mg C kg−1 soil ha−1) per unit of MBC (mg C kg−1 soil) over time using the laboratory incubation method. The rate of CO2 production from the soil samples after a 10-d incubation period was attributed to basal respiration. For this, a C-mineralization experiment was conducted for 10 d. The moisture content of 50 g air-dry soil was adjusted to 60% of water-holding capacity prior to its incubation in an air-tight plastic jar at a constant temperature of 25 °C. The microbial metabolic quotient (qCO2) (h−1) was calculated by dividing basal respiration by MBC.
2.6. Mineral N concentration 2.4. Soil microbial biomass carbon A total of 3 g of air dried soil was mixed with 30 ml of 2 M KCl and shaken for 1 h on a rotary shaker. The samples were then centrifuged at 1000g and subsamples were drawn using Swinnex filter holder fitted with glass microfiber filter for determination of nitrate-N by the automated cadmium reduction method (Baird and Bridgewater, 2017) using segmented flow analyzer (SEAL analytical, Norderstedt, Germany).
Microbial biomass C (MBC) was measured using the substrate-induced respiration (SIR) method (Horwath and Paul, 1994), in which 200 mg of glucose was dissolved in deionized water and added to 25 g dry soil to bring it to 60% of its water-holding capacity. The soil was incubated in plastic jars for 6 h at 25 °C, and the amount of CO2 that accumulated in the head space was measured by CO2 analyzer (WMA-5, 3
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30% of total SOC under the different treatments. It was found that 50 years of stubble retention and N fertilization significantly affected the stabilized C concentration in soil (Fig. 3A). Specifically, it was found that the management of the stubble and the N fertilization on the stabilized C concentration was the opposite to their effects on the charcoal C concentration. For example, the retention of stubble increased the stabilized C concentration by 18.6% compared to the SB treatments. Similarly, application of N fertilizer at a rate of 30 kg ha−1 increased the stabilized C by 18%, with 90 kg ha−1 increasing it by 26%. Furthermore, the effect of SR on C stabilization was more pronounced when it was coupled with NT. In contrast to stabilized C, no significant differences were observed for stabilized N between the various treatments (Fig. 3B). Regarding the C:N ratio of this fraction, it was observed that retention of stubble resulted in a significantly higher C:N ratio (13.5) compared to the SB treatments (11.4) (Fig. 3C).
2.7. Statistical analysis Data were statistically analyzed using three-way analysis of variance (ANOVA) procedure of SPSS version 25. The ANOVA and leastsignificant-difference (LSD) were calculated to compare variable means under different treatments. 3. Results 3.1. Soil total C, N stocks and C:N ratio After 50 years of management, the SOC stock in the top 0.1 m of the soil profile was significantly affected by both tillage and N fertilization. Specifically, the SOC stocks as average of N fertilization rates increased 19.9 Mg ha−1 in CT to 20.9 Mg ha−1 in NT, while SOC as average of tillage system increased from 18.5 Mg ha−1 in N0 to 20.8 Mg ha−1 in N30 and to 21.9 Mg ha−1 in N90, with the increase at N90 representing a 18% increase (Fig. 1A). In contrast, residue management did not significantly affect the SOC stocks, with the average SOC in the SR treatment (20.5 Mg ha−1) being similar to that in the SB treatment (20.3 Mg ha−1) (Fig. 1A). Overall, the treatment with the highest SOC stock was NT-SR-N90 (22.9 Mg ha−1), which was statistically at par with NT-SR-N30 and NT-SB-N90 and was in contrast to CT-SB-N0 and CT-SR-N0 which had the lowest SOC stocks (18.2 Mg ha−1). Thus, the greatest increases in SOC occurred at higher rates of N fertilization, with the use of NT in the absence of fertilization resulting in a more modest increase in SOC. Indeed, after 50 years, the use of NT together with the application of 90 kg N ha−1 and stubble retention resulted in a 22.2% increase in SOC stock when compared to the CT-SB-N0 treatment. Not only did management practices alter SOC, but also TSN. Again, the largest increase in TSN stock was observed upon the application of N fertilizers, with TSN increasing from an average of 1.05 at N0 to 1.32 Mg ha−1 at N90, with this representing a 25.7% increase (Fig. 1B). In addition, the retention of stubble also significantly (P < 0.001) increased the TSN stock in the soil, from an average of 1.14–1.26 Mg ha−1 (Fig. 1B). Also, the tillage regime was found to influence TSN stock, being 1.18 Mg ha−1 in the CT and 1.23 Mg ha−1 under NT treatments. For the C:N ratio, tillage practices alone did not cause significant differences. However, both stubble management and N fertilization significantly affected the C:N ratio of the soil. Specifically, the average C:N ratio changed from 16.3 for SR to 17.8 for SB (Fig. 1C), which was associated with the increase in TSN in soils where stubble was retained. In a similar manner, the average C:N ratio changed from 17.6 for N0 to 16.7 for N90 (Fig. 1C).
3.4. Soil microbial biomass carbon (MBC) After 50 years of management, it was found that MBC was significantly affected by tillage practices, stubble management and N fertilization. There was a positive and significant interaction between the tillage and stubble management; combinations of SR with CT, and N fertilization with CT, all resulting in higher MBC. It is interesting to note that the use of CT increased MBC (286 mg kg−1) compared to NT (243 mg kg−1), especially in the SR treatments (Fig. 4). Not only MBC was influenced by tillage, but also by stubble retention. It was found that the retention of stubble resulted in 30% higher MBC than in soils with stubble burned treatment. Similarly, N fertilization had a significant and positive effect on MBC values increasing from 234 mg kg−1 in N0 treatment to 274 mg kg−1 at 30 N ha−1 and 285 mg kg−1 at 90 kg N ha−1 (Fig. 4). 3.5. Metabolic quotient In the present study, we found a significant interaction between stubble management and N fertilizer application; however, 50 years of NT practice did not affect the metabolic quotient of the soil (Fig. 5). It was found that the metabolic quotient was significantly higher (2.99 × 10−3 h−1) in the SB treatments compared to that in the SR treatments (2.15 × 10−3 h−1) (Fig. 5A). The comparatively high additions (but low losses) of labile C in the SR treatment also means that more C is sequestered in the soil in stubble retained system; thus, this system contributes less to atmospheric CO2 than stubble burned system. We observed higher SOM accumulation under the SR than SB treatments. Furthermore, N fertilization significantly reduced the metabolic quotient of the soil, being significantly higher in the N0 treatment (3.80 × 10−3 h−1) than the N90 treatments where it was ca. 50% lower (1.7 × 10−3 h−1) (Fig. 5B). Indeed, even the application of 30 kg of N ha−1 significantly reduced the metabolic quotient (2.13 × 10−3 h−1). Interaction of N fertilization and stubble management was also significantly affected the metabolic quotient of the soil (Fig. 5C).
3.2. Charcoal C concentration (carbon remaining after treatment with H2O2 and HNO3) Overall, the charcoal C concentration ranged from 12 to 22% of SOC under the various treatments. However, the charcoal concentration of soil was found to be significantly affected by the stubble management practices, but not by either tillage or N fertilization (Fig. 2A). The burning of the stubble tended to increase the charcoal C concentration, with the highest value being 4.8 g kg−1 for the SB treatment with NT. Indeed, the charcoal C concentration was generally 3.3–4.8 g kg−1 in the SB soils but 2.3–3.3 g kg−1in the SR soils, being an average of 35.7% higher. No significant difference in charcoal total N concentration was recorded under different treatments (Fig. 2B). Finally, burning of stubble resulted in significantly (P = 0.002) higher charcoal C:N ratio (6.7) relative to the SR treatments (5.1) (Fig. 2C).
3.6. Nitrate-N concentration It was observed that the NO3−-N concentration of the soil that was collected after the harvest of wheat crop was affected by both stubble management and N fertilization. Tillage and N interaction effect on soil nitrate-N concentration was also found significant. It is interesting to note that nitrate-N concentration of SB treatment (19.8 mg kg−1) was significantly higher than the SR treatment (14.5 mg kg−1) (Fig. 6B). Application of N resulted in significantly higher NO3−-N concentration in soil, producing increases of 77% at 30 kg ha−1 and 233% at 90 kg ha−1 as compared to N0 treatment (Fig. 6A). Nitrogen application along with the CT practice maintained higher NO3−-N concentration in the soil in comparison to NT practice (Fig. 6C).
3.3. Soil carbon remaining after treatment with sodium hypochlorite (stabilized C) In the present study, we found that the stable C ranged from 18 to 4
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Fig. 2. Impacts of tillage, residue management and N fertilization on (A) charcoal C, (B) charcoal N and (C) C:N ratio. The vertical bars are the standard errors of the mean.
4. Discussions
reported that use of NT/SR and N fertilizer had not increased SOC stocks over time; although SR reduced loss of carbon in the 0–0.1 and 0–0.3 m layers and there was a trend towards reduced loss of carbon under NT in the 0–0.1 m. In a similar study at Central Ohio (USA), Duiker and Lal (1999) and Jha et al. (2017) reported a linear increase in SOC concentration with increasing rates of residue applied. Several authors have reported increase in SOC stock with balanced inorganic fertilization (Bharadwaj and Omanwar, 1994; Jha et al., 2014). Similarly, the largest increase in TSN was recorded with the highest rate of N application in our study (Fig. 1). Similar results were reported by Zhang et al. (2016) who examined the long-term impact of NT and stubble retention on soil N fractions after 22 years on a Chromic Cambisol. These authors also found that the use of no-till coupled with stubble retention was an effective management method for improving soil N stocks and increasing soil fertility. Similar findings have also been reported by others, with NT increasing the TSN compared to CT (Varvel and Wilhelm, 2011; Chen et al., 2009). Stubble burning significantly increased the charcoal C content of the soil (Fig. 2). This higher charcoal C concentration observed in the SB treatments was presumably due to the formation of char material due to long-term practice of stubble burning. During stubble burning, a fraction of stubble and SOM is converted into charcoal, a relatively stable C form. High levels of charcoal C resulting from repeated historical burning of grasslands, open woodlands, and agricultural crop stubbles have also been reported in soils from Australia (Skjemstad et al., 2002). Charcoal C is abundant in dark‐colored soils, affected by frequent vegetation burning, thus likely contributing to the highly stable aromatic components of SOM (Schmidt and Noack, 2000). Similar results have also been reported by Skjemstad et al. (2002), who reported that charcoal C constitutes 35% of SOC in US soils, while Lehmann et al. (2008) reported an average charcoal concentration of 33% of SOC in
Analysis of soil samples collected in 2018 from Hermitage site indicated that the use of NT, SR, and N fertilization had increased SOC stocks in the top 0.1 m of the soil compared to CT, SB, and no nitrogen fertilizer application (Fig. 1). Wang and Dalal (2006) also reported treatment effects in the 0–0.1 m depth were interactive and maximum SOC sequestration was achieved under the NT with SR and N fertilization. Our findings are in general agreement with those of Dalal et al. (1991) who studied the same site at Hermitage but after only 20 years of NT. These authors reported that residue retention increased organic C at 0–0.025, 0.025–0.05 and 0.05–0.1 m depths in soil under NT. Effects of fertilizer N application on organic C were primarily confined to 0–0.025 m depth although significant differences could be measured also in the 0–0.05 m layer. Dalal et al. (2011) further reported that after 40 years of treatment imposition, crop stubble and N fertilizer interactively increased SOC stocks at 0–0.10 m depth. They reported that fertilizer N application had no significant effect on SOC stocks when crop stubble was not retained. When crop stubble was retained, increasing rate of N fertilizer application resulted in higher SOC stocks. We also observed slightly higher SOC stock in SR than SB after 50 years of cropping, although the difference was insignificant. Our present findings are also in accordance with Mazzoncinia et al. (2016), for example, who reported a 22% increase in SOC (and TSN) content to a depth of 0.3 m in continuous NT treatments after 28 years on a Typic Xerofluvent. In the present study, the positive effect of N fertilization on SOC stocks was due to an increase in net primary productivity (Alvarez 2005; Christopher and Lal 2007; Ramirez et al.,2012). Yield response to N application (90 kg N ha−1 y−1) ranged from 9.50 to 24.9% under different treatments over N omission treatment (Dalal et al., 2011). Page et al. (2013) while analyzing time series data of Hermitage trial 5
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Fig. 3. Impacts of tillage, residue management and N fertilization on (A) stabilized C, (B) N concentrations and (C) C:N ratio. The vertical bars are the standard errors of the mean.
oxidizable C giving a good indication of the SOC fraction that has been stabilized over a period of time. Zimmermann et al. (2007) reported that isolation of C fraction after oxidation with NaOCl is a more suitable procedure for obtaining an operationally-defined stable organic matter fraction from soil. MBC was significantly affected by tillage practices, stubble management and N fertilization (Fig. 4). In the present study, CT had significantly higher MBC as compared to NT. This is in agreement with previous observations that tillage exposes SOM and increases mineralization, with mineralization of SOM occurring more rapidly in the CT
Australian soils. Long term practice of stubble retention and N fertilization significantly increased stabilized C in soil (Fig. 3). Collins et al. (2000) also reported that no-till increased resistant C by 20% in the surface 0.2 m at the three different sites in the US Corn Belt derived from both forest and prairie vegetation. The use of NaOCl has been proposed to remove less protected SOC to obtain a chemically resistant (stable) SOC fraction (Kleber et al., 2005; Mikutta et al., 2005). Indeed, such oxidative treatment may mimic biodegradation of the less protected SOC (Plante et al., 2004; Mikutta et al., 2006), with measurement of NaOCl
Fig. 4. Impacts of tillage, residue management and N fertilization on microbial biomass C (MBC) of the soil. The vertical bars are the standard errors of the mean. 6
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Fig. 5. Impacts of tillage, residue management and N fertilization on metabolic quotient (MQ) of the soil. In (A), values are the mean across N and tillage practices. In (B), values are the mean across residue management and tillage practices. In (C), values are the mean across all tillage practices. The vertical bars are the standard errors of the mean.
significantly affected by both stubble management and N fertilization (Fig. 6). Burning of stubble resulted in higher NO3−-N concentration in soil in comparison to SR treatment. Heat generated during fire induces the chemical oxidation of SOM, thereby altering C and N transformations. Choromanska and DeLuca (2002), while working on forest mineral soil which was exposed to fire, also reported lower basal respiration rates and lower concentrations of MBC, potentially mineralizable N, soluble hexose sugars, and NH4+-N, but higher NO3−N concentrations than in soil not exposed to fire. DeLuca et al. (2006) also reported charcoal produced due to forest fire significantly increased the nitrification potential, net nitrification, gross nitrification, but decreased the solution concentrations of plant secondary compounds (phenolics). Higher NO3−-N in fertilized treatments was due to conversion of added NH4+-N to NO3−-N. Higher NO3−-N concentration under the CT practice was probably due to higher organic matter mineralization under the CT as compared to NT practice, as shown by the higher microbial quotient in the former. After 50 years of management; N fertilization significantly affected SOC and TSN stocks. The retention of stubble increased TSN and stabilized C fraction, and increased the microbial biomass carbon in soil. The microbial metabolic quotient was significantly higher when stubble was burned and in the absence of N fertilization. Burning of stubbles significantly increased charcoal C concentration in soil.
than the NT treatments (Sarker et al., 2018). Sarker et al. (2018) also recorded higher MBC and MBN in the CT treatments in comparison to NT treatments. Moreover, differences in the availability of labile C and mineral N, in particular nitrate-N into the systems, may have also stimulated the growth of soil microbial population to enhance SOM decomposition (Chen et al., 2014). These results agree with findings from other long-term tillage studies, that soil disturbance increased the decomposition of SOM in surface layers due to higher microbial biomass and activity (Dalal and Chan, 2001), which in turn increased nutrient cycling (Bimüller et al., 2016). Geisseler and Scow (2014), while analyzing datasets from 64 long-term trials from around the world, also reported that mineral fertilizer application led to a 15.1% increase in the MBC as compared to unfertilized control treatments. They also noted that mineral fertilization increased SOC concentration, a factor contributing to the increase in MBC with mineral fertilization. The microbial metabolic quotient (respiration-to-biomass ratio), or qCO2, conceptually based on Odum's theory of ecosystem succession, is widely used as an index of ecosystem development and disturbance. Compared to SOC itself, qCO2 is likely more responsive to short term soil changes. A lower qCO2 is indicative of improved soil biophysical conditions resulting from amended soil with organic matter (Powlson and Jenkinson, 1981), but a higher qCO2 is indicative of soil degradation under intensive land use (Masciandaro et al., 1998). Stubble retention and N fertilization had significantly lowered microbial metabolic quotient of soil in comparison to SB and no N application (Fig. 5). These findings give an indication that the N application lowers C turnover while promoting microbial health in soils in the long term. These effects were probably due to improved microbial habitats and alleviated environmental stresses. Nitrate N concentration in soil after the harvest of wheat was
5. Conclusions After 50 years of management, application of N fertilizers were found to significantly increase SOC stocks in surface 0.1 m of the soil profile. The application of nitrogen fertilizers also increased total soil nitrogen as did stubble retention, but tillage did not influence the total 7
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Fig. 6. Impacts of tillage, residue management and N fertilization on nitrate-N concentration in the soil. In (A), values are the mean across all tillage and residue management practices. In (B), values are the mean across all N and tillage practices. In (C), values are the mean across all residue management treatments. The vertical bars are the standard errors of the mean.
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Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements Pramod Jha thanks the Department of Education, Australian Government for financial support received through Endeavour Research Fellowship (Recipient ID No. 6510-2018).
Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.geoderma.2019.113996. 8
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