Effect of cover plants on soil C and N dynamics in different soil management systems in dwarf cashew culture

Abstract

Inappropriate soil management practices are one of most important constraints on cashew production in Northeast Brazil. This work aimed to evaluate the changes in soil C and N dynamics caused by different management of cover plants (spontaneous or leguminous) in dwarf cashew culture in Northeast Brazil. The evaluated management systems for control of spontaneous vegetation included: disc harrowing (DH), mechanical mowing (MeM), manual mowing (MaM), leguminous cover crop (LEG), leguminous cover crop+mulch of Carnaúba palm (LEG+m), and application of herbicide (HERB). The floristic composition, plant density, biomass production, and the nutrient content of the spontaneous plants that grew between cashew rows were evaluated. Total soil C and N, mineral- and organic-N, light organic matter fraction, microbial biomass, CO2-C emission rates and humic substances were determined. HERB treatment promoted a substantial change in the floristic composition of spontaneous species. Richness index was highest in the LEG treatment in the fifth year of the experiment. Dry matter amount of spontaneous plants was quite similar in MeM, MaM, LEG, and LEG+m treatments, with average values of 2245kgha−1. DH and HERB treatments reduced dry matter by 69 and 80%, respectively. The MeM system resulted in the highest soil C stock, whereas the DH treatment tended to intensify the mineralization process of soil organic matter due to soil disturbance. LEG treatment resulted in the highest mineral-N contents. Our results suggest that the maintenance of soil organic C levels in DH and MeM treatments is strongly dependent on maintenance of light organic matter levels. Soil from treatments which favored input of organic matter (e.g., LEG, LEG+m and HERB) presented greater C-humic/C-fulvic acids ratio, indicating the predominance of humic acids. Our findings reinforce that cover plants (e.g., spontaneous vegetation and leguminous species) play an important environmental role, contributing to organic matter and nutrient cycling in the soil. Management systems that promote soil covering and the use of leguminous species increased soil organic matter stocks.