Soil C saturation in tropical agricultural systems: soil fertility, climate, texture and N controls on SOM fractions

Nature-based solutions for C sequestration in tropical croplands are paramount strategies in a changing climate. Soybean-maize-forage intercropped systems associated with soil acidity alleviation and nitrogen (N) fertilization effectively accumulate carbon (C) in weathered soils. However, the soil saturation status and the controls of C stabilization up to 40 cm soil depth into particulate (POM) and mineral-associated organic matter (MAOM) fractions are poorly understood in tropical croplands. This study tested the hypothesis of C saturation in the MAOM fraction by comparing field and published data focused on Brazilian tropical soils. We assessed the N fertilization effect on C stabilization in POM and MAOM pools, and climate, soil texture, and chemical attributes controls on POM and MAOM formation in tropical croplands using a machine-learning Random Forest (RF) modeling approach. Our findings do not support C saturation in the MAOM regardless of contrasting soil contents of silt plus clay and depths. C in the MAOM and POM fractions were not affected by N fertilizer. However, legume inclusion in the system resulted in higher total soil C and lower soil C:N ratio compared with N fertilization, which indicates that C dynamics differ whether synthetic or organic-N forms are applied to tropical croplands. Our RF model showed robust predictive performance for the MAOM but poorer for the POM fraction. Total organic carbon (TOC), total N, silt plus clay, phosphorus (P) and soil depth, zinc (Zn), cation exchange capacity, and TOC covariates were the most important variables for predicting MAOM and POM fractions, respectively. Our findings show, besides the well-known effect of calcium, that other nutrients such as P, Zn, manganese and copper play a key role in either MAOM or POM formation. Our work provides key insights on C saturation status in tropical croplands and land-management C sequestering potential, and N, climate, soil chemical attributes and texture controls on C distribution into MAOM and POM fractions up to 40 cm soil depth in tropical conservation agricultural systems. We advocate for upscaling C saturation concept nationally and further research investigating the role of climate and micronutrients contribution on MAOM and POM formation in broad scenarios and N fertilizer responsiveness for C stabilization in tropical croplands.