Clay-to-carbon ratio: An overlooked but pivotal mediator of soil nitrogen mineralization and availability

Abstract

The supply of nitrogen (N) to plants depends largely on soil organic matter content, but clay has a stabilizing effect on organic matter, protecting it from microbial attack, which may ultimately affect soil N mineralization rate. The clay-to-carbon ratio has recently been selected as an indicator of soil organic matter status. However, whether the clay-to-carbon ratio is relevant for assessing soil gross N mineralization rate (GNM) remains uncertain. By analyzing 1851 observations from 420 ¹⁵N-labelled studies, we found a significant and negative relationship between the clay-to-carbon ratio and both GNM and NH₄⁺-N concentration globally and across soil layers, land uses and climatic regions. Decreased clay-to-carbon ratio accelerated GNM via increasing soil microbial biomass, total N, and fungal abundance. The effect of soil pH, aridity and temperature on GNM is mediated through the clay-to-carbon ratio. Higher soil pH, aridity and temperature inhibited GNM by increasing the clay-to-carbon ratio. Thus, the higher the clay content, the more organic matter is required to ensure a high N supply to plants, especially in arid soils with high pH. Overall, incorporating the clay-to-carbon ratio into next-generation conceptual models as a pivotal mediator of GNM improves predictions of soil N supply globally.