Bedrock nitrogen inputs support litter nitrogen fixation and temperate forest ecosystem fertility

Abstract:

Nitrogen (N) is one of the most frequently limiting nutrients to terrestrial ecosystem productivity worldwide. As atmospheric carbon dioxide concentrations continue to rise, progressive N limitation is expected to constrain the ability of terrestrial ecosystems to store additional C, making an understanding of N inputs to terrestrial ecosystems increasingly important. In temperate forests, rock reservoirs and biological N fixation (BNF) represent two significant, but poorly characterized, inputs of bioavailable N. Recent research has demonstrated that bedrock can provide a substantial amount of ecosystem-available N in moderate-to-high relief areas with N-rich sedimentary bedrock. In these same ecosystems, asymbiotic BNF performed by heterotrophic microbes in plant litter can provide an additional N input of up to ~2 kg N ha^-1 yr^-1. Here, we tested the hypothesis that rock N inputs support increased litter BNF via enhanced ecosystem N fertility. We measured rates of BNF along with rock, soil, foliage, and litter chemistry across sites varying substantially in rock N concentrations (from 32 to 800 ppm N). The sites are dominated by Douglas fir and share similar climates and landscape positions (eroding slopes), yet display marked increases in foliar and soil N content as a function of rock N concentrations (foliar: R²=0.18, p<0.001, soil: R²=0.50, p=0.001). We found a significant positive correlation between rock N content and litter BNF rates (R²=0.11, p=0.0035), with rates of BNF at sites with greater than 400 ppm N in bedrock more than double rates of BNF at sites with lower than 400 ppm N in bedrock (p<0.001). These patterns could not be explained by increases in other rock-derived nutrients such as phosphorus or molybdenum, as neither of these known BNF controls increased over the rock N gradient. We found declining foliar lignin:N ratios with increased rock N, suggesting that rock N inputs can increase litter quality, supporting greater microbial activity, including BNF.