Simple measurements in a complex system: soil community responses to nitrogen amendment in a Pinus taeda forest

Microorganisms regulate the decomposition of soil organic matter and the flow of plant‐available nutrients. In a temperate pine forest in North Carolina, USA, where nitrogen (N) had been experimentally added for eight years, we combined DNA‐based measures of fungal and bacterial biomass and composition with measures of seven extracellular enzymes (ecoenzymes). These measures were then correlated with soil chemistry. Our goals were to evaluate the relative sensitivity and repeatability of multiple structural and functional measures of community organization to long‐term N deposition. Measurements were conducted at three litter/soil depths, corresponding to the litter, Oa, and mineral A horizons (max depth 10 cm) with ten spatial replicates per experimental plot. Soil chemistry differed significantly with soil depth and N amendment. Total biomass (soil DNA), as well as fungal and bacterial biomass (measured by qPCR), was greatest in the litter horizon and declined significantly with depth. Ecoenzyme activity patterns also changed with soil depth, transitioning from high levels of C, N, and P metabolizing activities in the litter horizon to increased oxidative activities at the lower depth. Under N amendment, soil pH decreased and nitrate concentrations increased in all three soil horizons. Correspondingly, the estimated microbial C use efficiency decreased in N‐amended soils at all depths, despite differences in microbial biomass, community composition, and soil chemistry. Overall, bacterial composition was most responsive to nitrogen amendment, but the taxonomic context of the response varied with soil horizons in conjunction with shifts in soil chemistry and enzyme activities. While a single measure that would incorporate all of the C, N, and P metabolizing activities did not emerge, many measures correlated with each other, and/or with depth and/or N amendment. Cycling of C and N between the atmosphere and the soil is a key regulator of global biogeochemical cycles and climate (Chapin et al. 2009, Bier et al. 2015, Bradford et al. 2016). Models using microbial parameters to estimate rates of C and N flow through soil systems have been developed that attempt to predict the influences of increased temperature, elevated atmospheric CO2, and N deposition on these rates (Finzi et al. 1998, Hogberg et al. 2006, Warby et al. 2009, Lovett and Goodale 2011, Bernal et al. 2012, Wieder et al. 2013, Mooshammer et al. 2014, Duran et al. 2016, Greaver et al. 2016, Sinsabaugh et al. 2017, Averill and Waring 2018, Ballantyne and Billings 2018). A challenge for