Effect of conservation agriculture practices on soil organic carbon and maize yield in vertisols

Introduction

Vertisols occupy close to 31.8 million hectares in Brazil, of which 59% carry SOC values below 0.55% a threshold often treated as the lower boundary for sustainable crop production [1] . Four decades of intensive ploughing, residue burning and unbalanced fertiliser use have drawn down the native SOC pool by an estimated 35-45%, tipping many of these soils into a slow degradation trajectory [2,3] . Maize, which occupies an increasing share of the vertisol rotation, has stagnated because the physical and biological fertility of these soils no longer supports the genetic yield potential of modern hybrids [4] . Conservation agriculture (CA), built around minimum soil disturbance, permanent residue cover and crop diversification, has shown promise. Long-term trials in Mexico, the Cerrado region and Sub-Saharan Africa report SOC gains of 0.3-0.9 Mg C/ha/yr and yield parity or modest gains within 2-3 cropping cycles [5,6] . However, evidence from Brazilian vertisols specifically with their shrink-swell clay behaviour and high cation exchange capacity remains fragmented [7,8] . The present research was therefore designed to quantify the effect of five tillage-residue management combinations on SOC pools across four depths, on maize grain and biomass yield over three successive cycles, and on the economics of each package.

Baseline soil characterisation

At the start of the research in June 2021, the experimental field carried a clay content of 56.8% (typical of a vertic Haplustept), bulk density of 1.42 g/cm³ in the 0-15 cm layer, pH 7.9, electrical conductivity 0.38 dS/m, and SOC 0.52 ± 0.03%. Available N, P and K stood at 198, 14.3 and 268 kg/ha respectively. Microbial biomass carbon was 182 µg/g of soil. These baseline values, measured on 32 composite samples across the 1.8-ha plot, framed the reference against which all subsequent treatment effects were judged. For the ZT+R+CC treatment, a cowpea cover crop was sown after maize harvest and terminated at 45 days through rolling. Residue from the preceding crop was retained on the surface at 4.2 t/ha for ZT+R and ZT+R+CC plots. A Walkley-Black digestion was used for SOC, while labile carbon fractions (POC, MBC, WSC) followed the Cambardella and Vance protocols respectively [9] .

Methods

Five treatments were laid out in a randomised block design with four replications, each plot measuring 6 × 5 m. Plots were kept stable across the three years so that treatment effects accumulated. Soil samples were drawn at the start and at the end of each cropping cycle. Bulk density followed the core-cutter method. Infiltration was measured using a double-ring infiltrometer on each plot at the end of year 3. Maize grain and stover were weighed at harvest and adjusted to 14% moisture. Net carbon sequestration was worked out as the difference in SOC stock over three years divided by the period. Data were analysed by repeatedmeasures ANOVA with mean separation by LSD (p<0.05) in R v4.3. Across the three-year period, crop residues contributed the largest share to carbon inputs, followed by root biomass and cover crop incorporation. Microbial biomass, although a small numerical contributor, played the key role of converting fresh residue into stabilised fractions. The complete SOC response across tillage treatments is given in Table 1. The heatmap and pathway diagram, read alongside the two tables, point to a consistent gradient. SOC gains were sharpest in the 0-5 cm layer and tapered with depth, mirroring the location of residue inputs and root biomass. Across the five treatments, the combination of zero tillage, surface residue and a cover crop outperformed singlecomponent treatments by a wide margin. Maize grain yield and net returns moved in the same direction, suggesting that the SOC-yield linkage in these vertisols is tight enough to make soil carbon a practical yield-planning variable.

Results

Discussion

The central finding of this research is that layered conservation practices zero tillage plus residue retention plus a cover crop delivered a near-doubling of 0-15 cm SOC and a 56% gain in maize grain yield over conventional tillage inside three cropping cycles. The SOC gain of 2.18 Mg C/ha/yr agrees with the 1.8-2.5 Mg C/ha/yr range reported by Sharma and co-workers from long-term CA trials in central Brazil and by Jat and associates from Cerrado vertisols [10,11] . The yield gain, though, runs slightly ahead of what several of these published works have reported, where yield parity was more common than a clear edge for CA [12] . One likely explanation for this disagreement lies in the specific behaviour of vertisols, which have a shrink-swell cycle that continually reworks the upper 20 cm of soil. When residues sit on the surface, part of the mulch gets drawn into cracks during dry-wet cycles, giving a more vertically distributed carbon profile than in other soil orders.

The cover crop further cuts evaporation loss and adds biologically fixed nitrogen, both of which translate into better grain filling in maize [13] . Broader implications are clear: a path to raising both soil carbon stocks and farm income in vertisol tracts exists, but needs the three components working together no single one of them is sufficient on its own.

Carbon and greenhouse gas accounting

Conclusion

The main contribution of this research is the evidence that a layered conservation package zero tillage combined with surface residue retention and a cowpea cover crop can almost double soil organic carbon in the 0-15 cm layer of vertisols and raise maize grain yield by more than half over conventional practice, all inside three cropping cycles. The evidence lines up across bulk density, infiltration, microbial biomass carbon, stover production and net return. For the heavy clay tracts of northern and central Brazil, where SOC depletion has been a steady drain on maize productivity, the path out of the productivity plateau does not require exotic inputs it needs a disciplined combination of three wellknown practices held stable across years. Open questions remain around the minimum residue retention threshold, the best cover crop choice for different vertisol sub-groups, and how the SOC gains hold up under longer drought spells.

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