Soil phosphorus stoichiometry drives carbon turnover along a soil C gradient spanning mineral and organic soils under rice cultivation

Background/Question/Methods

Soil carbon (C) cycling is linked to the availability of nutrients like nitrogen (N) and phosphorus (P).  However, the role of soil P in influencing soil C turnover and accumulation is poorly understood, with most models focusing on C:N ratios based on the assumption that terrestrial ecosystems are N limited.  To determine the effects of N and P availability on soil C turnover, we compared soil respiration over the course of a growing season in four adjacent rice fields with 5%, 10%, 20% and 25% soil C.  In each of these fields, plots were established to test the effect of N additions on plant growth, using control and N addition treatments (80 kg N/ha urea).  Although soil P was not manipulated in parallel, prior work has shown soil P concentrations decline markedly with increasing soil C content.  Soil respiration was monitored using static chambers at biweekly intervals throughout the growing season, along with porewater dissolved organic C and ammonium.  Soils were collected near the end of the growing season, and tested for total C, N, and P, extractable N and P, pH, base cations and trace metals.  Soil DNA was also extracted for 16S rRNA sequencing to profile microbial communities. 

Results/Conclusions

Soil N additions significantly increased CO₂ flux and soil C turnover (seasonal CO₂ flux per unit soil C) in 5% and 10% C fields, but not in 20% or 25% C fields.  Soil C content was closely related to soil N:P stoichiometry, with N:P ratios of ca. 12, 16, 24, and 56 respectively in the 5, 10, 20 and 25% C fields.  Seasonal CO₂ fluxes (per m²) were highest in 10% C soils.  However, soil C turnover was inversely related to soil C concentrations, with the greatest C turnover at the lowest values of soil C.  Soil C turnover showed stronger relationships with soil chemical parameters than seasonal CO₂ fluxes alone, and the best predictors of soil C turnover were soil total and extractable N:P ratios, along with extractable P alone. Our results show that soil P availability and stoichiometry influence the turnover of soil C, even where primary producers are clearly limited by N.  Prior work has suggested these contrasting patterns in nutrient limitation may arise due to stoichiometric differences among plants and soil microbes.  We hypothesize that differences in soil carbon turnover may in part reflect shifts in metabolism of microbial communities associated with stoichiometric variation in soils.