Biochar, N and P interaction drives C estabilization across soil organic matter fractions in sandy soils

Sandy soils in tropical regions pose significant challenges for enhancing soil organic matter (SOM) stocks. While management practices involving biochar (Bc) coupled with nitrogen (N) and phosphorus (P) fertilization offer potential for increasing carbon (C) within SOM fractions, the complex interactions between Bc, N, and P remain poorly understood. This study assessed C concentrations in particulate (POM) and mineral-associated organic matter (MAOM) in a sandy soil, with and without Bc, N, and P application, under Urochloa ruziziensis cultivation. The experiment utilized a randomized complete block design in a 2x2x2 factorial arrangement with four replicates. Rhizospheric ¹²C and ¹³C within POM and MAOM were quantified using isotope ratio mass spectrometry (IRMS). In the absence of Bc, N application alone stimulated C-POM loss, suggesting a rhizosphere-driven stimulus for P mineralization. Conversely, Bc application increased total C-POM and C-MAOM across all treatments, with the notable exception of total C-POM when both N and P were applied, indicating accelerated biochar decomposition (reduced biochar-derived C). Our findings suggest that P availability governs C-POM preservation under N fertilization. While Bc promotes C accrual across SOM fractions, N addition in the presence of P may mitigate these gains within the POM fraction, warranting further investigation into these biogeochemical trade-offs.