N:P re-mineralization ratios across forest ecosystems

Background/Question/Methods

Decomposition of leaf litter is the primary mechanism whereby nutrients are recycled in forests. Given the pervasiveness of nitrogen (N) and phosphorus (P) limitation in terrestrial ecosystems, elucidating patterns by which organic N and P are mineralized or immobilized in microbial biomass is crucial to establishing biome level controls on plant nutrient availability. In aquatic systems, nutrient re-mineralization proceeds at the characteristic Redfield ratio of planktonic biomass (i.e., molar ratio of 16N:1P). In terrestrial ecosystems, recent research has determined the N:P of soil, litter, microbes and leaves across biomes, but the N:P of re-mineralization is yet to be studied in a complementary way. Here, we address the question: does mineralization of N and P proceed at ratios that are predictable based on the N:P of litter across forest ecosystems? To address this question, we performed a meta-analysis of N and P mineralization from leaves and needles in forest ecosystems and included 112 studies, 511 litterbag sequences, and 3225 total observations. We compare re-mineralization ratios across temperate and tropical systems to elucidate patterns across latitudes using three techniques: arithmetic means via a Wilcoxon signed rank test, linear regression, and cumulative distribution functions.

Results/Conclusions

Across global forest ecosystems, the N:P of mineralization corresponds closely with the N:P of the litter substrates. The mean initial molar N:P were 46:1, 30:1, 58:1; whereas the mean N:P of mineralization were 46:1, 30:1, and 59:1 for global, temperate, and tropical forests, respectively. The distribution of data highlighted thresholds of 42:1 in temperate ecosystems and 63:1 in tropical ecosystems, between N limitation (N:P of mineralization < litter N:P at N:P < threshold) and P limitation (N:P of mineralization > litter N:P at N:P > threshold) in both temperate and tropical regions. Our analysis integrates two biogeochemical theories: a Redfield-based model where the N:P ratio of the litter matches the mineralization N:P ratio; and the McGill and Cole model of decomposition where P mineralization is higher in P limited ecosystems (low latitude areas) and N mineralization is higher in N or energy limited ecosystem (higher latitudes). Overall, our results support the Redfield model, in which microbes do not alter litter N:P during net mineralization. Conversely, plants affect the N:P of mineralization by resorbing nutrients prior to leaf fall.