Anthropogenic N Deposition Increases Soil C Storage by Decreasing the Extent of Litter Decay: Analysis of Field Observations with an Ecosystem Model

Abstract

Recent meta-analyses of experimental studies simulating increased anthropogenic nitrogen (N) deposition in forests reveal greater soil carbon (C) storage under elevated levels of atmospheric N deposition. However, these effects have not yet been included in ecosystem-scale models of soil C and N cycling and it is unclear whether increased soil C storage results from slower decomposition rates or a reduced extent of decomposition (for example, an increase in the amount of litter entering slowly decaying humus pools). To test these alternatives, we conducted a meta-analysis of litter decomposition data. We then used the results from our meta-analysis to model C and N cycling in four sugar maple forests in Michigan using an ecosystem process model (TRACE). We compared model results testing our alternative hypotheses to field data on soil C storage from a 17-year N deposition experiment. Using data from published litter decomposition studies in forests, we determined that, on average, exogenous N inputs decreased lignin decomposition rates by 30% and increased cellulose decomposition by 9%. In the same set of litter decomposition studies increased exogenous N availability increased the amount of litter entering slowly decaying humus pools in a manner significantly related to the lignocellulose index of decaying litter. Incorporating changes to decomposition rates in TRACE did not accurately reproduce greater soil C storage observed in our field study with experimentally elevated N deposition. However, when changes in the extent of decomposition were incorporated in TRACE, the model produced increased soil C storage by increasing the amount of litter entering the humus pool and accurately represented C storage in plant and soil pools under experimental N deposition. Our modeling results and meta-analysis indicate that the extent of litter decay as humus is formed, rather than slower rates of litter decay, is likely responsible for the accumulation of organic matter, and hence soil C storage, under experimental N Ó 2012 Springer Science+Business Media, LLC 450 deposition. This effect should be incorporated in regional to global-scale models simulating the C balance of forest ecosystems in regions receiving elevated N deposition.

Microbial Mechanisms Mediating Increased Soil C Storage under Elevated Atmospheric N Deposition

“…Meanwhile, the leaf litter N concentration has increased by 25%, and inorganic N concentrations in the forest floor have increased by 288% (3,5). With the use of field observations and a biogeochemical simulation model, we further determined that chronic N deposition has decreased the extent of microbial decay, thereby leading to an accumulation of soil organic matter (6). In combination, these observations indicate that a microbial mechanism underlies a decline in litter decay under experimental N deposition, which has enhanced soil C storage (ϩ10%) (3,7).…”

Responses of soil enzyme activity and microbial community compositions to nitrogen addition in bulk and microaggregate soil in the temperate steppe of Inner Mongolia

“…A three-years of N addition experiment suggested that N addition increased root and shoot biomass by 37 and 23%, respectively, without significant impact on decomposition, causing the increase in soil C by approximately 15% [80]. Other researches related to the response of plant community structure to N addition have suggested the negative influences on plant community and richness, possibly due to the acidifying effect of nitrogen increase [59,69,75]. Since soil enzyme activity and microbial community compositions influencing nutrient transformation between soil and plants, the studies on their responses to nitrogen input can be particularly important in understanding the underlying mechanisms of nutrient cycles.…”

Contrasting effects of ammonium and nitrate inputs on soil CO2 emission in a subtropical coniferous plantation of southern China

“…N input can promote or inhibit the rate and extent of soil organic matter decomposition through changing the activity and composition of microbial community (DeForest et al 2004), as well as the chemical property of organic matter (e.g., C/N, lignin/N, alkyl C/O-alkyl C) (Whittinghill et al 2012). Along with the continuous N input, NO 3 − and NH 4 + ions are rapidly assimilated by plants and soil microbes; subsequently, the N input stimulates soil microbial activity and decreases litter's C/N ratio leading to increases in litter decay and soil CO 2 emission (Knorr et al 2005).…”

“…In some grassland and forest ecosystems, low level of N addition increases litter return and stimulates soil microbial activity because it satisfies the N demand of soil microorganisms to decompose the relatively small pool of available labile C substrates with low N contents (Fang et al 2014b); however, high level of inorganic N inputs suppresses the synthesis of lignolytic enzymes by some fungi due to the decrease in organic matter decay extent (Cusack et al 2011;Whittinghill et al 2012). Also, because NO 3 − assimilated by plants and microbes has to be firstly reduced to NH 4 + , this energy-intensive process consumes more C fixed by photosynthesis and releases additional CO 2 (Gavrichkova and Kuzyakov 2008;Tischner 2000).…”