Soil properties after conversion to conservation agriculture from ridge tillage in Southern Malawi

Journal of Arid Environments 127 (2016) 7e16

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Soil properties after conversion to conservation agriculture from ridge tillage in Southern Malawi H.R. Mloza-Banda a, *, C.N. Makwiza b, M.L. Mloza-Banda c a

Crop and Soil Sciences Department, Faculty of Agriculture, University of Malawi, P. O. Box 219, Lilongwe, Malawi Agricultural Engineering Department, Faculty of Agriculture, University of Malawi, P. O. Box 219, Lilongwe, Malawi c Department of Soil Management and Care, Faculty of Bioscience Engineering, Ghent University, Coupure Links 653, Ghent B-9000, Belgium b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 21 November 2014 Received in revised form 1 November 2015 Accepted 3 November 2015 Available online xxx

A study was carried out to assess some of the likely early impacts on soil properties in small-scale farmers' fields following conversion to conservation agriculture (CA) from annual ridge tillage (RT). Five fields under CA applied for 2 years (CA2 yr) and 5 years (CA5 yr), respectively, were paired with adjacent RT fields with similar soil type and maize genotypes. Soils samples from the 20 fields were taken at depths of 0e10 and 10e20 cm and evaluated for physical and chemical characteristics. The results of analysis of variance and mean comparison showed that soil bulk density and total porosity (PORt) largely remained the same between CA and RT fields. Soil structural stability (SI) was marginally greater under CA practices while RT appeared to sustain better soil aggregation and root-zone aeration characteristics (air capacity, AC). CA5 yr fields showed greater values for soil organic carbon (OC), total nitrogen, available phosphorous, and cation exchange capacity (CEC). Volumetric water content at field capacity (qfc) and relative water capacity (RWC) were consistently greater under CA applied for the different number of years. The most influential soil attributes with loadings within ten per cent of those with the highest loadings based on multivariate principal component analysis were qfc, qpwp, qs, upper critical bulk density (rbU), OC, OC-stock, SI, RWC, AC, qfc/PORt, CEC, and K. Thus, soil attributes found as the most important to discriminate either management system were related to volumetric water content, soil organic carbon fraction, airewater storage, and chemical qualities. These were considered suitable for future assessment of soil quality for monitoring land use change effects in similar agroecosystems. The study posits study of socio-ecology of mulch cover practices to attribute management effects on soil properties. © 2015 Published by Elsevier Ltd.

Keywords: Air-water storage Conventional tillage Land use conversion No-till Small-scale farmers Soil quality

1. Introduction Soil erosion and land degradation are major concerns in the agricultural lands of the dryland South, especially in the context of the lands used by small-scale farmers. Ridge planting, as a reduced tillage system, has been credited with increased erosion control and to offer benefits of reduced fuel, chemical, and labour inputs compared with conventional tillage systems (Gürsoy et al., 2011). In many regions of the world, the practice is characterized by a permanent row-inter-row configuration where the rows are in the same location every year (Shi et al., 2012). Use is made of tractordrawn matched-width equipment including planters, fertilizer

* Corresponding author. E-mail address: [email protected] (H.R. Mloza-Banda). http://dx.doi.org/10.1016/j.jaridenv.2015.11.001 0140-1963/© 2015 Published by Elsevier Ltd.

side-dressing equipment, sprayers, and harvest machinery. In Malawi, ridge cultivation consists of annually remaking 15e20 cm high ridges in the furrow of previous ridges. Plant residues are covered with inverted soil, removed, or burnt, and a weed-free seedbed is maintained for the first two months of crop growth until the soil is covered by the crop canopy (Mloza-Banda et al., 2010). While ridging can be accomplished using draft implements, small-scale farmers, who cultivate 90% of total arable land in Malawi, only have hand-held hoes that can only work at shallow depths of the soil. Cultivation to the same depth annually may result in gradual formation of a compacted horizon immediately below the cultivated layer. The hard pan prevents infiltration leading to ponding of water that accumulates, breaking ridges and eroding the soil (Douglas et al., 1999). Shaxson and Barber (2003) reported similar observations in Zambia and Tanzania where repeated tillage to the same depth caused subsurface pans of

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compacted soil at the base of the tillage layer that precluded penetration by roots or water. Conventional ridge tillage (RT) methods may thus create conditions that reduce the potential to conserve rainwater in agricultural fields. Yet, rainfall availability is prone to great temporal variability exacerbated by changes in climate (Fowler and Rockstrom, 2001). The variable rainfall becomes more acute in areas prone to weather anomalies such as the hinterland of Southern Malawi. The location is of typical mid-altitude rainshadow zones found mainly west of raised north-south oriented land massifs that traverse the country and along the Great East African Rift Valley. Farmers in Malawi and the region have since been encouraged to practice conservation agriculture (CA) in response to the challenge of temporal and spatial fluctuations in rainfall availability, land degradation and ardous tillage practices. The CA practices adopted for small-scale farmers in Malawi are the no-till mulch-based systems. They entail managing crop residue on the soil surface with no tillage, change to high maize plant density, fertilizer application and weed control amongst other inputs. Yield improvement equivalent to 3e6 t ha
1 of maize compared with 1 t ha
1 by CA has been attained by farmers (Ito et al., 2007). Although the contribution to the increase in yield from CA may be self-evident to some, soil characteristic benefits have not been widely studied on small-scale fields (Giller et al., 2009). While the emphasis on the importance of outcome is acknowledged, our study considered soil properties due to a desire for improved process understanding. Elsewhere in the region, crops are grown on conventionally tilled flat seedbed (Shaxson and Barber, 2003). The learning curve in the promotion and application of no-till, from annual RT, is thus considerably steep in Malawi and wherever there is yearly interchange of row and inter-row ridge configurations. It is thus critical in the short-term, to allow farmers and their agents understand impact of land use change in order to sustain their quest for climate-smart and workable land management practices. We hypothesized that studying soil attributes of contiguous fields under long term ridge cultivation and recently converted fields to CA, we may detect possible land use change effects on soil quality characteristics. The objectives of this study were therefore to assess and compare selected soil properties under CA following change from RT on the mid-altitude dryland plains of Southern Malawi. The goal was to characterize changes in soil properties and determine whether in the short term, CA significantly improved soil quality towards halting and reversing the degradation of agricultural soil. Data were collected from fields that had been managed by farmers with no scientific oversight by research scientists or technicians since conversion. The aim was for data to reflect the consequences on soil quality, of technical and local socio-economic drivers following land use conversion. The development of relationships amongst soil attributes may be a monumental task, in particular, where indicators and methods have not been validated for local soils. Venema (1991) provided the only country-wide land suitability and capability characteristics for Malawi that detailed important parameters like climatic conditions, soil orders, and parent materials. Later work (Snapp, 1998) provided soil fertility indicators validated for selected zones. Our study used these and other empirical guideline values considered optimal soil quality for maximum yield crop production and resource conservation given the advent of land degradation and climate changes the last three decades (Reynolds et al., 2007). It is recognized that thresholds of soil quality evidently differ depending on soil type, local climate, land use and other factors. Arshad and Martin (2002) suggested that rather than selection of critical limits for soil quality indicators, development of a set of guidelines that can help set limits for defined crop/environment situations may be more prudent given the complexities of yield response to critical soil parameter values.

2. Materials and methods 2.1. Site description The study took place on the Balaka Plains in Southern Malawi located at latitude 14 300 e15º200 S and longitude 34 400 e35º300 E and at a mean altitude of 625 m above sea level. The plains cover an area of over 210,000 ha that support 95,000 farm families under Balaka Rural Development Project. Venema (1991) described the landscape as consisting of dominant slopes at 1e6 per cent with soils derived from basement complex rocks (gneisses) and approximate Chromic Luvisols and Chromic Cambisols based on FAO classification. Temperatures are relatively high with the average minimum and maximum temperatures at 18.3  C and 28.6  C, respectively. The warmest months are October and November (32  C) at the instance of the onset of the rain season. The district is characterised as semi-arid area with single modal rain, varying between 400 and 800 mm per year (Fig. 1) with more than 90% of total rain falling between November and April. Balaka is predominantly in a rain shadow area with an average of 35 rainfall days per year and experiences frequent dry spells and droughts. Based on frequency analysis (RAINBOW software package 2.2, March 2006, Leuven, Belgium), the mean annual rainfall for a 20year period for this site was 828.3 (SD, 208.6 mm). The mean annual rainfall received in a wet year (with a 20% probability of exceedance) was 1103.8 mm and in a normal year (with an 50% probability of exceedance) was 840.8 mm. On the other hand, in a dry year (with an 80% probability of exceedance) it was 640.4 mm. Thus, five out of ten years that received below normal rainfall were dry years. Different crops grown by farmers in the study area under varying cropping systems are shown in Fig. 2. Other data used in the paper included socio-ecological information from farmers regarding crop and residue management practices (Mloza-Banda et al., 2010). 2.2. Field selection and soil sampling The study was conducted towards the end of the rainy season at harvest maturity of the maize crop in 2010. Ten fields managed by small-scale farmers were identified for soil sampling. These fields had been under CA for 2 or 5 years, respectively, and were previously under continuous annual RT. Fields had been managed by

Fig. 1. Mean monthly precipitation (mm) for a 20-year period (1990e2010) and average precipitation in wet, normal and dry years for Balaka Plains, Southern Malawi.

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MCR ¼ ð% sand þ % siltÞ=ð% clay þ % organic matterÞ

9

(1)

Bulk density (rb) was determined using methods described by Blake and Hartge (1986). Indices that describe the soil's mechanical resistance to root growth were calculated based on empirical relationships that distinguish a lower critical rb (rbL, Mg m
3), below which root-soil contact is insufficient for water retention and plant anchoring, and an upper critical rb (rbU, Mg m
3), above which root growth is severely impeded due to increasing soil strength (Reynolds et al., 2002) (Eqs. (2) and (3)):

Fig. 2. Field crops grown by farmers sampled in the study.

farmers with no scientific oversight by research scientists or technicians since conversion. Five fields were thus selected to represent each of the 2 or 5 years of CA, and were paired with adjacent RT fields for direct comparisons giving a total of twenty fields. The period of CA practice (in years) has been indicated in subscript. Care was taken that paired fields had matching soil types and were subjected to similar crop management practices for the respective durations of the CA practice. CA practices included no-till, mulching with the previous season's maize stover, and pre-plant and preemergence chemical weed control with Round-upTM (Glyphosate, 1.44 kg a. i. ha
1) and Harness™ (Acetochlor, 2.2 kg a. i. ha
1), respectively. Ridge-tillage practices performed by farmers comprised remaking 15e20 cm high ridges by hand hoeing while inverting the soil and burying previous crop and other plant residues. Farmers further routinely maintained a weed-free seedbed during the first two months of crop growth. All maize received identical fertilizer rate of 69 kg N ha
1 supplied as 100 kg of N:P:K ha
1 (23:21:0 þ 4 S) at planting and, 100 kg urea ha
1 (46% N) or 200 kg calcium ammonium nitrate ha
1 (CAN, 23% N), approximately three weeks after planting. Six bulk soil samples per field were taken from the 0e10 cm and 10e20 cm layers for the determination of gravimetric moisture content, pH (H2O), salt content using electrical conductivity (EC) (dS cm
1), sand (g kg
1), silt (g kg
1), clay (g kg
1), total organic C (%), total organic N (%), P available (ppm) and K extractable (ppm). In each layer, the samples were bulked per plot into one composite sample, thoroughly mixed, air-dried and ground to pass a 2 mm sieve before assay. Bulk density (rb, Mg m
3), total porosity (m3 m
3), and volumetric water content (m3 m
3) were determined on undisturbed soil samples taken from the 0e10 cm and 10e20 cm layers of each field using the core method (Cornelis et al., 2005). Soil samples for determination of aggregate stability were taken randomly from 6 replicates per plot from the 0e10 cm layer and gently mixed to obtain a composite sample. In RT fields, both bulk and core samples were taken in the ridge crest but cores were further obtained in the middle of the sampling depths.

2.3. Determination of soil physical properties Soil texture was determined by the Bouyoucos hydrometer method (Elementar, GmbH, Hanau, Germany) with particle size distribution derived according to the USDA agricultural textural classes. The modified clay ratio (MCR) was identified as a useful approximation (Singh and Khera, 2008) of soil erodibility from land-use transformations (Eq. (1)):

rbL ¼ 1:60
0:00468ðcl þ siÞ

(2)

rbU ¼ 1:83
0:00429ðcl þ siÞ

(3)

where cl (%) and si (%) are the clay and the silt content of the soil, respectively. To determine dry aggregate distribution (D'haene et al., 2008; Govaerts et al., 2009), 150 g of air-dried soil sample (<8 mm) was placed onto the top of the five sieves (2, 1, 0.5, 0.15 and 0.075 mm), and gently shaken at 30 oscillations per minute. Soil remaining on each sieve was collected, dried at 105  C in an oven and weighed. The material retained on each sieve gave the soil aggregate size distributions while aggregate stability was expressed by calculating the mean weight diameter (MWD, mm) and geometric mean diameter (GMD, mm) (Kemper and Chepil, 1965). Soil structural stability index (SI) was calculated based on the formula of Pieri (1992) given as (Eq. (4)):

SI ¼ ½1:72 OCðwt:%Þ=½ðclay þ siltÞðwt:%Þ  100

(4)

where SI is a soil structural ‘‘stability index’’, and (clay þ silt) is the soil's combined clay and silt content. 2.4. Determination of soil chemical characteristics Soil pH (1:1 soil: water) and electrical conductivity (EC, dS cm
1) in a 1:5 soil weight (g): distilled water (ml) suspension were determined. Cation exchange capacity (CEC) was measured at pH 7 by ammonium acetate extraction (Van Ranst et al., 1999). Total organic C was assessed by a wet digestion and colourimetric method (Anderson and Ingram, 1993). Carbon storage was estimated as: carbon stock (t ha
1) per soil layer ¼ carbon (%)  bulk density (Mg m
3)  layer depth (m)  10,000 (m2 ha
1). Available phosphorus (P) and exchangeable K were extracted according to Mehlich III (Mehlich, 1984) and analysed by inductively coupled spectrometry. The P and K data were correlated to and reported as Bray P (Bray and Kurtz, 1945) and as a 1 N ammonium acetate extraction (McIntosh, 1969), respectively. Total N was evaluated using the same methods for P but with P-nitrophenol added for colour development and subsequent analysis performed by means of an Atomic Absorption Spectrophotometer Model 200A (Elementar, GmbH, Hanau, Germany). 2.5. Determination of soil hydraulic properties Saturated hydraulic conductivity (Ks) was determined using the constant-head method (Cornelis et al., 2005). After Ks measurements, a subsample from the bottom of each core was used to determine gravimetric water content and water retention at selected suctions. Pressure chambers (Soilmoisture Equipment, Santa Barbara CA, USA) were used to subject subsamples to pressure heads of j ¼
33 kPa, which corresponds to field capacity (FC, qfc) and of j ¼
1500 kPa, the permanent wilting point (PWP, qpwp), respectively. Water content at saturation (qs) was set equal to the

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porosity of the undisturbed core. Plant available water capacity (PAWC) was the difference in water content between qfc and qpwp while air capacity (AC) was that between qs and qfc. The soil's capacity to store water relative to the total pore volume is given by the relative water capacity (RWC) which was determined as the proportion of qfc to qs (Reynolds et al., 2007). Soil airewater storage variables (qfc/PORt and AC/PORt) were derived from the proportion of qfc and AC to total porosity (PORt), respectively (Olness et al., 1998). 2.6. Statistical analysis Analysis of variance based on the Generalised Linear Model was used to test the statistical differences in soil properties using the SPSS 20 software (SPSS Inc., 2013, Chicago, Illinois, U.S.A.). An Omnibus Test compared the fitted model against the intercept-only model while the Wald ChieSquare tested the main effects of tillage practices, soil layers sampled and the interaction between tillage systems and depth. Data from contiguous CA and RT fields were paired in the analysis. The levels of significance of the mean differences, at 95% Wald Confidence Interval for Difference, were determined using the Bonferroni test, over other tests, given the number of pairs of means. The probability (P values) associated with pairwise comparisons of estimated marginal means are presented for the main effects of tillage systems. Standard errors are given to indicate how closely mean values reported approximate population means and n values are shown to assist in describing the confidence limits of the means (Webster, 2001). Mean values for respective depths and interactions have not been reported owing to largely non-significant differences between soil layers. Principal component analysis (PCA) using SPSS 20 software package was performed to identify principal directions in which data varied and to reduce dataset to interpretable size by identifying factors that contain most of the variance of the associated variables. PCA transforms variables to new uncorrelated variables, principal components (PCs) or factors that are extracted from a dataset by cross product matrix (Starkweather, 2011). In computational terms, PCs were found by calculating the eigenvectors and eigenvalues of the data correlation matrix. Those components with eigenvalues greater than one were extracted. To establish variables that had higher loadings (using Pearson's r  0.5) in the extracted components, a component matrix and its subsequent rotation with Kaiser, was derived. The rotated component matrix displayed component loadings for each variable. The principal components receiving high eigenvalues and variables with high factor loading were assumed as the variables that best represent system attributes. On the other hand, factor loadings (correlation values) greater than 0.5 were considered in explaining the components. In addition to grouping the soil characters, the sign of attributes' coefficients in each component represented the relationship between the characters. Pearson's correlation coefficient matrix was further used to describe relationships among properties of soils that had the highest weight in the PCA (Munoz et al., 2007). It was assumed that the variables having the highest correlation sum best represented the group. 3. Results and discussion 3.1. Soil physical properties According to the USDA classification scheme, the texture of soil samples from individual fields were principally clay loams to clay with some soil layers subtending sandy clay and sandy clay loams as shown in Figs. 3 and 4. Soils from RT fields had a significantly greater sand content and

lower silt and clay content as compared with CA fields (Table 1). Soil aggregates are expected to readily break down with loss of organic matter (OM) and consequently the finer particles are transported by erosion leaving behind coarse (sand) particles. This is perhaps enhanced under ridge cultivation with the apparent sifting of the soil as ridges are made annually or earthed-up during the season. Van Oost et al. (2006) observed that unlike water and wind erosion, whose effects are often dramatic and can be easily identified in the landscape, the extent and severity of tillage erosion only become apparent after several decades through variations in soil properties. Cultivation systems did not have significant influence over bulk density (rb) which varied from 1.53 to 1.59 Mg m
3 and 1.49e1.58 Mg m
3 for RT and CA fields, respectively. Differences between practices were more noticeable for contiguous RT and CA5 yr fields for the lower (rbL) and upper (rbU) critical rb indices. The significance of these indices, unlike rb, suggested that absolute values of soil properties may not capture the natural range of soil attributes. For fine-textured soils, Reynolds et al. (2007) suggested that upper limit rb for adequate root-zone aeration varies from 1.25 to 1.30 Mg m
3 and root elongation becomes severely restricted at rb 1.4e1.6 Mg m
3. Earlier work in Malawi showed average rb of 1.41 and 1.50 at 0e15 and 15e30 cm depths, respectively, under RT with samples obtained from furrows (interrows) (Douglas et al., 1999). The significant changes in rb between soil layers were attributed to cultivation induced compaction immediately below the crop ridge which is remade in the former interrow annually. In our study, samples for rb were obtained in the ridge crest. Most tillage operations are performed to decrease soil density within the disturbed zone and often studies have shown increase in rb under no-till during early stages. Long term results have however been variable with increases, decreases or similar rb values between zero tillage and conventional tillage (Strudley et al., 2008). CA5 yr practices marginally improved soil structural stability index (SI) to which OC and the soil's clay and silt content contribute (Table 1). However, SI for all sampled fields was far below a 5% threshold that indicates structurally degraded soil (Pierri, 1992). The determination of the stability of the soil is further obtained by aggregate analysis (Kemper and Chepil, 1965). Aggregate size distribution is important because of its influence on watertransmission status and as an index of erodibility (Van Oost et al., 2006). CA2 yr and RT practices did not significantly influence aggregate size distribution while aggregates between 1 and >2.00 mm were significantly higher in RT fields compared with contiguous CA5 yr fields (Fig. 5). RT fields, compared with CA5 yr fields, subtended higher values for both geometric mean diameter (GMD) and mean weight diameter (MWD) (Table 1). Dead roots and above-ground residues that each year are scraped into the furrow which becomes the new ridge under RT, may have served as additional substrate for soil microorganisms that effectively sustained soil aggregation shown to be comparable and/or better than under CA. Given that the effects of OC accumulation under CA were subtle, these results suggested a different pace of change in OC and aggregation. D'haene et al., (2008) described lack of correlation between MWD values obtained using various methods with texture and OC content. They attributed results to small range in soil texture and OC content. Alvaro-Fuentes et al. (2006) reported that, in some cases, total OC may not be sufficient to explain differences in aggregate stability. Daraghmeh et al. (2009) found that dry stability of aggregates from soil under reduced tillage was always lower than that of conventionally tilled soil. They suggested that conventional tillage conferred a lower stability on the soil against mechanical forces, active under e.g., tillage, rain splash and wind erosion. Conversely, as the wet stability is largely dependent on biological activity and OM, reduced tillage resulted in a higher stability against physico-

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Fig. 3. Particle size distributions and textural classification of soils in the 0e10 cm (d1) and 10e20 cm (d2) layers in individual contiguous fields (1e10) cultivated by CA2 years and ridge tillage (RT) systems.

Fig. 4. Particle size distributions and textural classification of soils in the 0e10 cm (d1) and 10e20 cm (d2) layers in individual contiguous fields (1e10) cultivated by CA5 years and ridge tillage (RT) systems.

Table 1 Mean comparisons of soil physical properties. Variable

Mean CA2

Sand (%) Silt (%) Clay (%) rb (Mg m
3) rbL (Mg m
3) rbU (Mg m
3) SI GMD (mm) MWD (mm 100 g
1) MCR

yr

36b 21a 43a 1.49a 1.32a 1.58a 2.92a 3.43a 1.45a 1.32b

SE

P value

RT 45a 18b 37b 1.53a 1.35a 1.60a 2.51a 3.46a 1.58a 1.66a

Mean CA5

2.31 1.37 2.26 0.04 0.013 0.012 0.40 0.018 0.09 0.116

0.000 0.028 0.008 0.355 0.093 0.092 0.309 0.177 0.165 0.004

yr

37.3b 19.7a 43.0a 1.58a 1.30b 1.56b 3.77a 3.45b 1.54b 1.27b

SE

P value

2.10 1.73 1.40 0.03 0.011 0.010 0.38 0.018 0.091 0.082

0.000 0.050 0.001 0.872 0.000 0.000 0.000 0.000 0.000 0.053

RT 45.3a 16.3b 38.3b 1.59a 1.34a 1.59a 1.70b 3.52a 1.86a 1.59a

CA2 yr, conservation agriculture for 2 years; CA5 yr, conservation agriculture for 5 years; RT, annual ridge tillage; rb, bulk density, rbL, lower critical bulk density; rbU, upper critical bulk density; SI, structural stability index; GMD, geometric mean diameter; MWD, mean weight diameter; MCR, modified clay ratio; means with different superscripts in the same row for contiguous fields for each variable indicate the meaningful difference at 0.05% confidence level (Bonferroni test, p < 0.05); n ¼ 10 for means and n ¼ 20 for SE.

chemical forces related to changes in water content and active under e.g., infiltration and evapotranspiration, slaking, swelling, and flocculation. They concluded that soil stability needs to be assessed in the context of the nature of disintegrating forces.

Values of modified clay ratio (MCR) were observed to be greater in RT fields than CA fields implying greater susceptibility to erodibility (Table 1). This suggested that RT may have resulted in good structural distribution as determined by dry sieving but the

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Fig. 5. Mean aggregate-size fractions for ridge tillage fields paired with CA2 years and CA5 years fields, respectively. Different letters on each bar within aggregate size fraction indicate the meaningful difference at a ¼ 0.05%.

structural components could have been weak to resist water slaking as projected by MCR. Lobb et al. (1999) suggested erosivity of a given tillage operation and the erodibility of the cultivated landscape, as antecedents of the process of tillage erosion. In our study, the MCR gave an indication of both aspects with greater values in RT fields being symptomatic of course texture (Table 1) probably following removal and redistribution of top soil given annual shift of ridge position and pervasive low organic matter (Table 2). The subject of erodibility is important because the critical time for satisfying the soil cover threshold is the time of crop emergence (Erenstein, 2003). At the onset of the season, crops generally produce limited/no cover whereas there are attendant challenges to the use of mulching practices under CA (Mloza-Banda et al., 2010) implying potential for soil erodibility. 3.2. Soil chemical properties Soil pH only changed by 0.3 towards greater alkalinity between RT and CA fields (Table 2). It has been reported that pH changes very little within landscape units of few hectares (Shukla et al., 2004). Moreover, changes in pH are further attributed to the parent material and climate under which soil formation takes place. Nonetheless, soil pH is one of the factors which affects the equilibrium levels as well as the rate of decomposition of OM in soils by modifying the local edaphic environment and so restrict rapid OC biodegradation characteristic of conventional tillage (VandenBygaart et al., 2003). Soil salt content measured as electrical conductivity (EC) did not vary between corresponding RT and CA fields in conformity with the observation that the primary contributor to soil conductivity in non-saline fields is soil texture which did not differ. Conductivity

values in the contiguous fields were within the lower crop sensitivity range (2e4 dS m
1) for plant growth (Abrol et al., 1988). Under rainfed conditions however, detrimental effects of salinity can be hastened under untimely dry spells leading to heightened osmotic tension of saline soil solutions and premature soil and plant desiccation. This effect may be pervasive under RT where rain falling on the ridge crests tends to rapidly run off and infiltrate into the lower interrow spaces as reported by Shi et al. (2012). They found the highest soil water content in the interrows, lowest in the crests, and intermediate in the shoulders of ridges. CA practices brought about significant increases in OC, N, P, and CEC levels over RT practices (Table 2). CA5 yr fields compared to RT fields, subtended higher levels of K and OC-stock. Earlier work has suggested that such differences probably also reflect the concentration of fertiliser nutrients in the top layer on zero tilled land (Horne et al., 1992). Although the stratification phenomenon has been characterised as the striking feature of no-till, Shi et al. (2012) found that there was a significantly higher total OC in no-till than mouldboard plough tillage on an equivalent mass basis. In our study, the mean values of OC were significantly greater under CA practices (1.09e1.33%) than RT (0.54e0.8%) (Table 2). Snapp (1998) used a critical value of 0.8% for evaluating OC in southern Malawi which was barely exceeded in all soils from farmers' fields under CA in our study. Recent studies reported changes from 0.04 to 0.08% for N and 0.62e1.04% for OC after 5 years of zero tillage in loamy sand and sandy loam soils elsewhere in Malawi (Ngwira et al., 2012). Carbon stocks differences were found only between CA5 yr and RT fields with the former fields subtending mean value of 31 Mg C ha
1 (Table 2). Shi et al. (2012) considered that the impacts of no-till on OC stock vary with time whereas VandenBygaart et al. (2003) reported an inverse relationship between OC content and the effect of

Table 2 Mean comparisons of soil chemical properties. Variable

Mean CA2

pH EC (dS m
1) OC (%) OC-stock (t ha
1) Total N (%) Available P (ppm) Exchangeable K (ppm) CEC (cmol (þ) kg
1)

yr

6.02a 4.20a 1.09a 24.44a 0.101a 35.6a 129.35a 15.24a

SE

P value

RT 5.69b 4.32a 0.80b 18.33a 0.061b 12.7b 97.35a 13.38b

Mean CA5

0.14 0.18 0.15 3.51 0.013 3.29 27.1 0.302

0.016 0.508 0.050 0.082 0.002 0.000 0.238 0.000

yr

6.29a 3.48a 1.33a 31.37a 0.082a 66.3a 224.93a 19.16a

SE

P value

0.21 0.39 0.12 2.73 0.007 3.09 39.51 0.52

0.103 0.331 0.000 0.000 0.000 0.000 0.004 0.000

RT 5.95a 3.86a 0.54b 12.74b 0.040b 15.5b 109.81b 15.64b

Note: CA2 yr, conservation agriculture for 2 years; CA5 yr, conservation agriculture for 5 years; RT, annual ridge tillage; EC, electrical conductivity; OC, total organic carbon; CEC, cation exchange capacity; Means with different superscripts in the same row for contiguous fields for each variable indicate the meaningful difference at 0.05% confidence level (Bonferroni test, p < 0.05); n ¼ 10 for means and n ¼ 20 for SE.

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tillage on OC with gains due to adoption of zero tillage occurring mainly at OC levels of less than 45 t ha
1. They speculated that depleted old soils have more potential to sequester carbon compared to young soils rich in carbon. Increases in organic C with conservation management have previously been observed by other workers, although the results depended on soil type, cropping system, kind of crop residue, management, and climate (Corsi et al., 2012). Munoz et al. (2007) reported significant increase in organic C just after two years of conservation management in semi-arid Spain. Brejda et al. (2000) suggested assessing soil attributes every five years to allow differences among management systems to accumulate due to the relatively small changes in OC with time. It is further considered more appropriate to evaluate more labile fractions of OC to assess changes in soil quality during shorter time periods (Ngwira et al., 2012). Snapp (1998) suggested P levels of >13 ppm for evaluating soils in southern Malawi. For exchangeable K, values of 200 ppm were considered high and providing excellent reserves. In our study, soils from RT and CA2 yr fields generally fell short of these thresholds. Yet, it has been held that soils in Malawi are broadly K sufficient (Snapp, 1998) and the mineral element is not provided for in fertilisers contrived for most field crops. The traditional postulation where soils are held as exclusive supplier of K nutrition to crops should be re-examined in different zones to elucidate whether the inherent cause is mineral state or hydraulic property. Variability of extractable P has been recognized in Malawi soils. The wide range of fertilizer and agronomic management histories across the landscape of small-scale farmers' parcels of land are considered significant causes (Snapp, 1998). Soils from all fields (Table 2) barely exceeded the lower threshold for CEC, which for clay loams falls between 15 and 30 cmol (þ) kg
1 (Van Ranst et al., 1999). This could be a reflection of low turnover rate owing to mulch management under CA or rapid mineralisation rate under annual RT (Corsi et al., 2012). Duxbury et al. (1989) observed that the importance of OM to CEC increases as soils weather and change from 2:1 aluminosilicates (CEC ¼ 15e30 cmolc kg
1 soil) to kaolinite and amorphous oxides of Fe and Al. Further, that most of the CEC in kaolinitic soils is associated with OM and maintaining high OM levels is especially important in tropical and sandy soils. Soils with CEC values 20 cmol (þ) kg
1 are considered structurally resilient and to buffer the inputs of acidity and rainfall more effectively than those that have low buffering capacity even for soils with an OC content <2.5% (Corsi et al., 2012). A high CEC is regarded as favourable as it also contributes to the capacity of soils to retain plant nutrient cations.

13

3.3. Soil hydraulic characteristics Although total porosity largely remained the same for contiguous cropping systems, significant differences in volumetric water content occurred for qfc and qpwp (Table 3). This concurs with Bescansa et al. (2006) who showed that available water capacity under no-till was greater than under reduced or minimum tillage with most of this difference being due to retention between j ¼
33 and j ¼
50 kPa. They argued that unlike soils in more humid climates, water content in semiarid soils remains under field capacity for most of the growing season. Thus, water retention between qfc and qpwp is considered critical in the evaluation of soil hydraulic properties. Reynolds et al. (2002) reported that PAWC should be >0.20 m3 m
3. PAWC values for contiguous CA and RT fields in Table 3 were well below the suggested range although CA5 yr fields exhibited greater PAWC compared with adjacent RT fields. Reynolds et al. (2007) suggested that rain-fed agricultural soils that allow desirable water and air contents (for maximum microbial activity) more frequently and for longer time periods are where 0.6  RWC  0.7. RWC values from CA and RT fields were within this range with CA fields subtending greater values (Table 3). It was noted that despite slightly higher aggregation under RT, soil hydraulic properties were not correspondingly greater compared with CA fields. It is probable that plant residues that are mixed into the soil through RT favour quick degradation processes that do not subsequently improve water infiltration and retention (Corsi et al., 2012). RT fields appeared well aerated having values of greater than 0.17 m3 m
3 considered ideal AC value while CA fields were within the minimum air capacity of 0.10e0.15 m3 m
3 for adequate nearsurface root aeration (Reynolds et al., 2007). The optimal balance between soil water content and soil aeration for crop growth has further been considered in terms of the ratios of qfc/PORt and AC/PORt, respectively (Olness et al. 1998). The former is an index of the balance between soil water holding capacity and aeration. Contiguous CA2 yr and RT fields were below the suggested threshold of 0.66 for qfc/PORt while CA5 yr fields and corresponding RT fields were above the threshold value. The AC/ PORt ratio was higher than the ideal of 0.34 for CA2 yr and RT fields while CA5 yr fields reduced the ratio towards the suggested threshold. Invariably, qfc/PORt and AC/PORt did not clearly distinguish the cultivation practices. Hydraulic conductivity (Ks) has been used to classify a soil according to the permeability classes proposed by O'Neal (1949). Soils from all CA and RT fields fell within permeability classes of moderately rapid (Ks ¼ 17.4e34.7 ms
1). However, RT fields exhibited higher Ks than adjacent CA2 yr fields but this difference

Table 3 Mean comparisons of soil airewater variables. Variable

Mean CA2

PORt (m3 m
3) Hd (%) qs (m3 m
3) FC (qfc, m3 m
3) PWP (qpwp, m3 m
3) AC (m3 m
3) PAWC (m3 m
3) RWC qfc/PORt AC/PORt Ks (m s
1)

yr

0.439a 4.65a 0.506a 0.354a 0.232a 0.152b 0.121a 0.699a 0.51a 0.51a 29.0b

SE

P value

0.016 0.45 0.006 0.009 0.009 0.005 0.004 0.012 0.060 0.039 1.64

0.355 0.000 0.374 0.002 0.011 0.000 0.242 0.000 0.240 0.690 0.006

RT 0.424a 3.03b 0.501a 0.325b 0.209b 0.176a 0.116a 0.649b 0.58a 0.49a 33.5a

Mean CA5

yr

0.402a 4.99a 0.510a 0.359a 0.237a 0.151b 0.121a 0.701a 0.82a 0.35b 30.83a

SE

P value

0.01 0.44 0.007 0.017 0.015 0.012 0.004 0.025 0.045 0.016 1.98

0.872 0.007 0.050 0.025 0.085 0.043 0.007 0.033 0.333 0.000 0.736

RT 0.40a 3.81b 0.497b 0.322b 0.213a 0.175a 0.109b 0.647b 0.78a 0.42a 31.50a

Note: CA2 yr, conservation agriculture for 2 years; CA5 yr, conservation agriculture for 5 years; RT, annual ridge tillage; PORt, total porosity; Hd, soil humidity; qs, volumetric water content at saturation, FC, water content at field capacity, PWP, water content at permanent wilting point; AC, air capacity; RWC, relative water capacity; PAWC, plant available water capacity; Ks, saturated hydraulic conductivity; Means with different superscripts in the same row for contiguous fields for each variable indicate the meaningful difference at 0.05% confidence level (Bonferroni test, p < 0.05); n ¼ 10 for means and n ¼ 20 for SE.

14

H.R. Mloza-Banda et al. / Journal of Arid Environments 127 (2016) 7e16

was not evident in CA5 yr fields. Higher Ks values have been observed with loosening the soil under conventional tillage. Increase in macropore conductivity is commonly observed in zero tillage with residue retention that is considered to arise from increased number of biopores but divergent results have been obtained in conductivity response to zero tillage (Strudley et al., 2008). 3.4. Relationship among soil attributes The thirty soil attributes were subjected to factor analysis by principal component (PC) methods and factor rotation by the varimax method (Starkweather, 2011). Variables that loaded on two components or which by themselves created a component with less than three variables were eliminated. Four components were derived with eigenvalues greater than one and these were retained for interpretation. The PCA model explained 88% of the total variance in the data for all soil attributes (Table 4). The first component, with the most explanation for the changes, had variance of 32.28%, in which the characters of FC, PWP, qs, rbU, and PAWC existed. The component principally represented variables related to volumetric soil water content. The second component, consisting of OC, OC-stock and SI included 22.25% of the total changes. According to the characters of this component, it summarised the OC fraction. The third component explained soil water holding and transport parameters and accounted for 17.76% of the total changes. It consisted of RWC, AC and Ks. The fourth component, accounting for 15.98% of the total changes, consisted of qfc/PORt, CEC, K and AC/PORt. The component mainly grouped soil chemical attributes and airewater storage parameters. Andrews and Carroll (2001) further suggested that the variables with loadings within ten per cent of those with the highest loadings can be considered as the ones best representing the system attributes, and could be selected as the most sensitive topsoil quality indicators for the studied soil. Such variables were FC, PWP, qs, and rbU in PC1, OC, OC-stock and SI in PC2, RWC and AC in PC3 and, qfc/PORt, CEC, and K in PC4 (Table 4). The results of PC analysis confirmed the suggestion that single attribute indicators, such as identified through analysis of variance, do not fully reflect soil quality response to changes in management.

Table 4 Factor loading values of four principal components extracted after varimax rotation and proportion of variance explained. Variable or statistic

Component 1

FC PWP

0.979 0.937 qs 0.919 rbU
0.884 PAWC 0.840 OC
0.099 OC-stock
0.068 SI
0.162 RWC
0.248 AC 0.307 Ks
0.468 qfc/PORt
0.148 CEC
0.143 K
0.100 AC/PORt 0.455 Rotation sums of squared loadings Total 4.842 % of variance 32.280 Cumulative % 32.280

2

3

4


0.080
0.049
0.178 0.080
0.126 0.973 0.968 0.958 0.275
0.258 0.063 0.105 0.410 0.368
0.110

0.072 0.191
0.329 0.308
0.203 0.129 0.156 0.120 0.894
0.874
0.647 0.246 0.227 0.265
0.397

0.055 0.038 0.022 0.026 0.076 0.100 0.123 0.075 0.036
0.050 0.151
0.843 0.773 0.766 0.658

3.338 22.253 54.534

2.664 17.758 72.292

2.397 15.982 88.274

Extraction Method: Principal Component Analysis. Rotation Method: Varimax with Kaiser Normalization. Rotations converged in five iterations.

On the other hand, correlated soil attributes do not change independently with changes in management but respond as a group, integrating many complex interactions among chemical and physical soil processes (Brejda et al., 2000). In the area of no-tilled soils, Munoz et al. (2007) reported that OC, CEC, volumetric water content, and available K, also evident in our study, were among fifteen variables that discriminated soils under conventional and no-till management regimes. Similarly, OC, BD, AC, FC, and Ks, identified with our study, were among eighteen variables that explained >79% of variability in measured soil properties when notill, with and without residues and rotations, were compared with conventional tillage (Shukla et al., 2006). Correlation analysis was used in PCA to compute Pearson's correlation coefficient matrix for the properties of soils that had the highest weight in the PCA (Table 5). The analysis showed a significant correlation among 52 of the 105 soil attribute pairs (P < 0.05). The study of the correlations between variables allowed understanding of the relationship between the studied soil properties across different principal components. For instance, with correlation coefficients, r > 0.70, rbU, showed strong correlation with variables (qs, qfc, qpwp and PAWC) that described volumetric water content in PC1. Thus, the correlation matrix showed variables in PC2 that correlated with some of the observed variables that did not display strong correlations with PC1. Further, data in Table 5 augments the principle that each component remains uncorrelated with all of the preceding components. It is evident, for example, that coefficients for OC were completely uncorrelated with coefficients for volumetric water content. This observation suggested that volumetric water content was less affected by the insufficient OC additions (Table 2) following change in land use and management practices. In any case, the functions of OC almost always affect several different properties and engage in multiple reactions given the effect of OC on soil structure (aggregate stability), on cation exchange capacity and buffer capacity of soils, and on the soil's water holding capacity (Corsi et al., 2012). 3.5. Soil cover The results of this study generally showed better soil water content at field capacity (qfc) and a positive trend for RWC both of which could contribute to time to reach wilting point under CA practices. This is consistent with farmers' observations of resilient crop stands especially at the instance of dry spells like that occurred for 15 days between December 2009 and January 2010 within the period of this study (Mloza-Banda et al., 2010). In addition, these results are in accord with assertions that the benefits derived from mulch tillage depend on the agroecological zone (Fowler and Rockstrom, 2001). Where marginal or erratic rainfall or drought is a problem, the major benefits are increased moisture capture and retention under mulch practices. The amount of residues produced at each sampled field or the annual production for each farmer was not captured in our study. Fowler and Rockstrom (2001) observed that the minimum quantity of mulch needed for short-term moisture conservation benefit is 5 t ha
1. This may be difficult to achieve due to multiple uses for residue. Smaller quantities of residue (2e3 t ha
1) are considered satisfactory to improve soil physical properties in the long-term if applied each year. Residue retention is considered a challenge often based on a blanket perception that farming systems in the region are all but predominantly mixed crop-livestock systems where most crop residues are grazed in situ by livestock or carted to the homestead for tethered or penned animals (Mapfumo and Giller, 2001). None of the households sampled for study owned ruminant livestock that would predispose maize-based crop residue to competitive use. The study district has a cattle herd of 7648 against

H.R. Mloza-Banda et al. / Journal of Arid Environments 127 (2016) 7e16

15

Table 5 Correlation matrix for highly weighted variables under principal component analysis.

AC/PORt

rbU OC OC-stock K SI

qs FC PWP PAWC RWC AC Ks CEC

qfc/PORt

AC/PORt

rbU

OC

OC-stock

K

SI

qs

FC

PWP

PAWC

RWC

AC

Ks

¡0.73 0.19 0.03 0.03 ¡0.41 0.06
0.23
0.16
0.10
0.26 0.24
0.23
0.13 ¡0.43

¡0.45
0.13
0.08 0.22
0.16 0.56 0.45 0.37 0.53 ¡0.44 0.44 0.10 0.23

0.20 0.19 0.19 0.30 ¡0.91 ¡0.83 ¡0.77 ¡0.76 0.53 ¡0.57 0.22 0.20

0.99 0.46 0.97 ¡0.31
0.17
0.13
0.23 0.41 ¡0.40
0.00 0.50

0.48 0.96
0.29
0.14
0.09
0.20 0.43 ¡0.41
0.03 0.51

0.43
0.20
0.06
0.01
0.18 0.34 ¡0.33 0.02 0.84

¡0.36
0.23
0.17
0.27 0.42 ¡0.41 0.07 0.47

0.90 0.83 0.84 ¡0.58 0.63
0.20
0.25

0.97 0.84
0.18 0.23 ¡0.42
0.11

0.67
0.07 0.13 ¡0.48
0.07

¡0.37 0.40
0.18
0.16

¡0.99 ¡0.33 0.35

0.29 ¡0.36

0.10

Correlation is significant at the 0.05 level (2-tailed) for coefficients in bold.

75,656 households and only 2% of the households owned cattle (Malawi Government, 2007). Such qualitative and quantitative information is needed to address the right questions and to integrate local and scientific knowledge before hastily cordoning-off innovations. Farmers in the study area typically bury crop residues during the ridging operation (Mloza-Banda et al., 2010). In some locations, the practice of leaving residues on the soil surface under CA, encountered the problem of subterranean termites that devoured the mulch early after application. Farmers' innovativeness came to the fore where they erected platforms in-field or off-field to protect residues that were transferred back to the fields towards onset of rains. The practice was at variance with the CA pillar of permanent soil cover that has critical influence on soil aggregate formation and stability or the diminution of post-season weed flora (Erenstein, 2003). The practice may thus explain low OC and other related properties observed under CA. In Malawi, mulch under CA practices has been generated as a byproduct from in situ production systems, typically maize crop residues from the previous harvest. Table 3 presented a range of crops that farmers grew in the study area with varying potential as source of vegetative biomass for mulching. However, crops were either produced on limited piece of land to provide sufficient mulch (Mloza-Banda et al., 2010) or were by nature prone to biomass weathering (Erenstein, 2003). It is evident nonetheless that farmers in the area, albeit few, were growing drought tolerant crops (sorghum, millet, pigeon peas) that ought to be compatible with CA and its prerequisite for non-fragile residues. The socio-ecology of mulch cover practices is considered essential in future studies related to soil quality following land use change. This will offer equal attention to both the inherent site characteristics and to the impacts of innovative management practices. 4. Conclusion The data presented give an indication of the short-term potential of a tropical clay loam soil to respond to change in management under small-scale farmer conditions that may be realized elsewhere in controlled experimentation. The study did not assume initial homogeneity of fields but it attempted to redress observations based on warily controlled field plots. The aim of this study was neither to assess validity of the selected attributes as soil quality indicators; rather for a range of properties and indices to assist in the appraisal of change in cultivation practice from ridge tillage to conservation agriculture in response to land degradation and climate variability. Analyses of variance procedures merely evaluated the significance of sources of variability in the measured

soil attributes. Principal component analysis was used as a typical stochastic approach to reduce the information in the measured variables into a set of weighted linear combinations of those variables in order to identify their underlying structure. Volumetric water content (qs, qfc, qpwp, and PAWC) and upper critical bulk density (rbU) formed the first principal group of variables while OC, OC-stock and stability index formed the second group. The third group comprised of relative water content, air capacity and hydraulic conductivity. The last group included cation exchange capacity, potassium concentration and airewater storage parameters, qfc/PORt and AC/PORt. With the capability to evaluate these properties on farmers' fields early in the adoption cycle of land use change, the ability of specific management is realised in similar and various soils and at varying scales, as a benefit for farmers and their agents. Acknowledgements The authors thank the International START Secretariat Grants for Global Environmental Change Research in Africa 2010 (Africa 2010e04) for financial support during conduct of this study. We thank staff and farmers of Machinga Agricultural Development Division and the Farm Input Diversification Programme, Malawi, who participated in this study. We thank L. Chirwa and B. Msukwa for field and laboratory assistance and to L. Fiwa for collation of rainfall data. We acknowledge Prof. Dr. ir. W. Cornelis, UNESCO Chair on Eremology, for leading us to the enriching field of research on no-tillage systems. We are also indebted to reviewers and editors whose comments added value to this manuscript. References Abrol, I.P., Yadav, J.S.P., Massou, F.I., 1988. Salt-affected Soils and Their Management. Food and Agriculture Organisation of the United Nations, Rome, Italy, p. 131. No. 39, FAO Soils Bulletin. pez, M.V., 2006. NoAlvaro-Fuentes, J., Arrúe, J.L., Cantero-Martínez, C., Gracia, R., Lo tillage, soil organic matter and soil structure: relationships and implications. In: mes rencontres Arrúe Ugarte, J.L., Cantero-Martínez, C. (Eds.), Troisie diterrane ennes du semis direct. (Options Me diterrane ennes: Se rie A. me minaires Me diterrane ens; n. 69). CIHEAM, Zaragoza, pp. 149e153. Se Anderson, J.M., Ingram, J.S.I., 1993. Tropical Soil Fertility and Biology: a Handbook of Methods, second ed. CAB International, Wallington, United Kingdom. Andrews, S.S., Carroll, C.R., 2001. Designing a soil quality assessment tool for sustainable agroecosystem management. Ecol. Appl. 11, 1573e1585. Arshad, M.A., Martin, S., 2002. Identifying critical limits for soil quality indicators in agro-ecosystems. Agric. Ecosyst. Environ. 88, 153e160. Bescansa, P., Imaz, M.J., Virto, I., Enrique, A., Hoogmoed, W.B., 2006. Soil water retention as affected by tillage and residue management in semiarid Spain. Soil Tillage Res. 87, 19e27. Blake, G.R., Hartge, K.H., 1986. Bulk density. In: Klute, A. (Ed.), Methods of Soil Analysis. Part I. Agronomy 9. American Society of Agronomy, Madison, WI., USA,

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