Soil aggregation governs physically protected carbon and nitrogen in coffee systems

Soil aggregates play an important role in reducing carbon (C) and nitrogen (N) losses from the soil, as they act as a physical barrier to soil organic matter decomposition. However, studies evaluating the occlusion of C and N in soil aggregates in coffee cultivation systems are lacking. In this study, we evaluated the effect of three coffee cultivation systems (monoculture, intercropping with banana, agroforestry system with grevillea) on aggregate distribution and occluded carbon and nitrogen stocks, using a native forest as a reference. Four plots were evaluated for each cultivation system, and soil was sampled at six depths (0-10, 10-20, 20-40, 40-60, 60-80, and 80-100 cm). Data were analyzed using linear mixed models, with the cultivation system as a fixed effect and sampling points as random effects. Soil samples were fractionated to obtain three aggregate classes: macroaggregates (2000-250 μm), microaggregates (250-53 μm), and silt + clay (53 μm). Macroaggregates and microaggregates were sonicated to quantify occluded C and N. Macroaggregates were predominant in all systems and depths, representing an average of approximately 78% of the total soil mass, with a reduction along the profile. The weighted mean diameter of aggregates decreased with depth and showed a positive relationship with the soil C content (r² = 0.68; p < 0.05). Overall, approximately 14.5 and 1.22 Mg ha^-1 (11% of C and N in whole soil) of soil carbon and nitrogen were physically protected within aggregates in the soil profile. Macroaggregates was the largest pool, which concentrated more than 50% of the occluded C and N up to 40 cm depth. Coffee cultivation systems promoted soil aggregation and physical protection of C and N similarly, indicating that conservation management and the continuity of organic matter inputs play a more decisive role in these processes than the diversification degree of the cultivation system.