Nitrogen addition changes grassland soil organic matter decomposition

Abstract

Humans have dramatically increased the deposition and availability of nutrients, such as nitrogen (N), worldwide. Soil organic matter (SOM) is a significant global reservoir of carbon (C); however, the effects of N enrichment on this large, heterogeneous C stock are unclear. Nitrogen has variable effects on the biological, chemical, and physical factors that determine SOM pool mean residence time; consequently, we predicted that N enrichment would have distinct effects on SOM pools, including the pool that is readily available for microbial decomposition, as well as the pools that have been stabilized against microbial decomposition via aggregate occlusion and mineral association. We addressed this gap in knowledge by measuring the effects of N addition on different SOM pools at five grassland experiments in the US Central Great Plains that participate in the Nutrient Network and have been fertilized for three or five years. Overall, N addition decreased microbial respiration of unoccluded OM by as much as 29 % relative to control plots, and consequently, decreased C loss from this pool. Furthermore, N addition tended to increase soil aggregation and C occlusion in large macro-aggregates. These results suggest that N addition will increase C sequestration by slowing the decomposition of SOM, as well as stabilizing SOM against microbial decomposition in aggregate-occluded pools. However, the effects of N on all pools studied varied among sites, possibly due to site variation in soil texture. Consequently, increased sequestration of soil C in response to N enrichment may not be universal across grasslands.

Chronic nitrogen fertilization and carbon sequestration in grassland soils: evidence of a microbial enzyme link

“…This also agrees with the findings from a long-term N fertilization experiment in meadow communities of alpine tundra, which suggest that under chronic N addition detritus plant material can move directly into stabilized mineral-associated organic matter pools (Neff et al 2002). Finally, our finding agrees with the observation that N fertilization can contribute to C accumulation and stabilization in small mineral-associated soil aggregates (Riggs et al 2015).…”

Soil carbon storage, respiration potential, and organic matter quality across an age and climate gradient in southwestern Greenland

“…Therefore, we interpret CO 2 production at the end of the incubation to be from a recalcitrant pool. It is possible to use microbial respiration data to model the size and decay rates of soil carbon pools (see Riggs et al 2015), but our analysis focuses on differences in soil respiration rates and the decay of these rates during the incubation period. We used Akaike's information criterion to identify the best (lowest AICc) and other plausible models (DAICc \ 2) for each sample.…”

Physico-chemical protection, rather than biochemical composition, governs the responses of soil organic carbon decomposition to nitrogen addition in a temperate agroecosystem

“…Soil organic carbon (SOC), which is the largest carbon reservoir in the terrestrial biosphere, contains two to four times the amount of carbon in the atmosphere. Consequently, either positive or negative changes of soil carbon storage, in response to N deposition, could lead to significant alteration in the carbon dioxide (CO 2 ) concentration of atmosphere (Riggs et al, 2015;Zak et al, 2017), and thus have implications for future global climate change.…”

“…Previous studies revealed that soil N input has the potential to decrease microbial decomposition of SOC and thereby increase soil carbon storage (Ramirez et al, 2012;Zhou et al, 2014;Riggs et al, 2015). Although this negative effect of N input on microbial decomposition of SOC seems paradoxical since the increased input of plant litter and root exudates induced by N enrichment may increase carbon availability to microbes (Liu and Greaver, 2010), a number of microbial mechanisms have been proposed to explain why microbial decomposition of SOC decreases with N addition (Riggs and Hobbie, 2016).…”

“…Thus, responses of SOC decomposition to N addition may be more strongly associated with the level of physicochemical protection (aggregate occlusion and mineral association) than with the biochemical composition of organic substrates. Although previous studies have documented the effects of N additions on the distribution of SOC in different functional fractions (Cusack et al, 2011;Riggs et al, 2015;Xu et al, 2016;Zak et al, 2017), very few studies examined the heterogeneous responses of SOC decomposition rates in different physical and chemical fractions to N enrichments (but see Neff et al, 2002;Cusack et al, 2010). Clearly, the mechanisms regarding these heterogeneous responses warrant investigation given their expected importance to understand soil carbon emissions in the context of increasing N deposition.…”