Fertilizer 15N tracing reveals compost-induced changes to mineral-associated organic matter and soil nitrogen pools

Abstract

Understanding how soil organic matter (SOM) fractions, such as mineral-associated organic matter (MAOM) and particulate organic matter (POM), respond to nutrient management practices is essential for improving soil health and advancing sustainability in agroecosystems. In particular, there is a need for strategies that sustain and enhance soil fertility while simultaneously reducing nitrogen (N) losses and greenhouse gas emissions. Co-applying compost and fertilizer has the potential to improve soil health by building SOM and increasing fertilizer N retention in the soil, which can be especially beneficial for low nutrient-demanding crops like olives. In a two-year field study conducted in a super-high-density olive orchard, we investigated the effects of compost application and N fertilization rate on SOM fractions down to 90-cm depth and on nitrous oxide (N₂O) emissions. Using ¹⁵N-labeled fertilizer, we traced fertilizer N in the soil over time. Compost application increased the concentration of carbon (C) and N in topsoil, including in MAOM and POM, with the largest effects occurring in the first year. We also observed greater concentrations of C and N within MAOM at deeper soil layers with compost, but compost did not increase the concentration of fertilizer N remaining in the soil over two years. Compost significantly reduced N₂O emissions, especially background emissions. These effects may have been mediated by increased soil C from compost application, particularly dissolved organic C, which may have driven SOM turnover, MAOM formation, and N₂O reduction. Together, these results suggest that compost application can be an effective strategy for sustainable nutrient management and building soil health, particularly in low-input perennial tree crop systems.