Cover crop and nitrogen fertilisation effects on soil organic carbon and phosphorus fractions in a silage maize-based conservation agriculture system.

Abstract

Purpose Cover crops (CCs) have been demonstrated to enhance soil organic carbon (SOC) and available phosphorus (P); however, there is a notable lack of long-term studies examining their effects on SOC and P fractions, particularly in South Africa. This research aims to understand the impact of nitrogen (N) fertilisation of silage maize in rotation with unfertilised CCs, including their combinations, on SOC and P fractions. Methods and materials First, a meta-analysis was done to assess how N-fertilised and unfertilised CCs affected SOC and available P under diverse conditions. A field trial experiment was also done to assess the impact of N fertilisation of silage maize in rotation with unfertilised (CCs), including their combinations, on (a) SOC fractions (b) soil P fractions, and (c) other chemical properties. The trial was established in 2003, and the CCs for the current study were established in 2011. The trial was a completely randomised block design, with nine CCs treatments in rotation with maize silage in three replicates, each split for N fertiliser (0 and 120 kg N ha⁻¹) in 2012. Soil samples were collected from 0-5, 5-10, and 10-20 cm depths using beater auger. Results and discussion The meta-data analysis showed that CCs planted as mono- and biculture improved SOC and available P, especially under legumes, except when mixed with broadleaves crops. It was also evident that CCs thrive and are more effective under tropical climates, particularly on loam soils. While SOC was higher under neutral pH, CCs increased available P in acidic soils. Nitrogen fertiliser application on CCs significantly increased SOC and available P than when it was not applied. From the field study, cover crops significantly affected total organic carbon (TOC) and coarse particulate organic carbon (coarse-POC) at 10 cm, fine-POC at 20 cm, and mineral-associated organic carbon (MAOC) only at 5 cm, with no effects on water-soluble organic carbon (WSOC). The TOC was higher under black oat and stooling rye + vetch. At the same time, POC fractions were higher under forage pea + black oats, stooling rye + vetch, black oats + vetch, black oat and stooling rye, and the MAOC was higher under forage pea, which also increased fine-POC at 10-20 cm when compared with the control. On the other hand, vetch greatly increased the total N. Nitrogen fertilisation had no effects on SOC fractions except for coarse-POC, which was decreased. Total cations were significantly increased by black oat at 10 cm and forage pea + black oat at 10-20 cm. Extractable calcium was not significantly affected by CCs, however, it was higher under black oat + vetch and vetch at 10 cm. Forage pea also increased Ca at 5-10 cm, and it was higher under forage pea + black oat at 10-20 cm. Cover crops decreased soil pH, especially vetch. As a result, P bound to Al and Fe was very high in all CC treatments than AP. Total P was higher under black oat + vetch, stooling rye + vetch and black oat with N fertiliser at 0-5 cm, CCs with and without N fertiliser except for black oat + vetch and forage pea without N fertiliser at 5-10 cm, and it was higher in all CCs in 10-20 cm. While extractable P was not significantly affected, CCs significantly increased soluble P. It was higher under forage pea + black oat, stooling rye + vetch, stooling rye and black oat at 5-10 cm, and forage pea + black oat at 10-20 cm. Conclusion The findings of the study imply that certain cover crops, like forage pea, black oat, and vetch, enhance SOC, N, and P levels while varying in effectiveness by soil depth. Vetch is especially beneficial in acidic soils, improving fertility, pH, and P retention while promoting carbon sequestration and sustainable nutrient management.