Anthropogenic N deposition increases soil organic matter accumulation without altering its biochemical composition

Abstract

Accumulating evidence indicates that future rates of atmospheric N deposition have the potential to increase soil C storage by reducing the decay of plant litter and soil organic matter (SOM). Although the microbial mechanism underlying this response is not well understood, a decline in decay could alter the amount, as well as biochemical composition of SOM. Here, we used size-density fractionation and solid-state C-NMR spectroscopy to explore the extent to which declines in microbial decay in a long-term (ca. 20 yrs.) N deposition experiment have altered the biochemical composition of forest floor, bulk mineral soil, as well as free and occluded particulate organic matter. Significant amounts of organic matter have accumulated in occluded particulate organic matter (~20%; oPOM); however, experimental N deposition had not altered the abundance of carboxyl, aryl, alkyl, or O/N-alkyl C in forest floor, bulk mineral soil, or any soil fraction. These observations suggest that biochemically equivalent organic matter has accumulated in oPOM at a greater rate under experimental N deposition, relative to the ambient treatment. Although we do not understand the process by which experimental N deposition has fostered the occlusion of organic matter by mineral soil particles, our results highlight the importance of interactions among the products of microbial decay and the chemical and physical properties of silt and clay particles that occlude organic matter from microbial attack. Because oPOM can reside in soils for decades to centuries, organic matter accumulating under future rates of anthropogenic N deposition could remain in soil for long periods of time. If temperate forest soils in the Northern Hemisphere respond like those in our experiment, then unabated deposition of anthropogenic N from the atmosphere has the potential to foster greater soil C storage, especially in fine-texture forest soils.

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

“…4). This idea is also supported by a recent study that showed the identical biochemical composition of mineral soil organic matter between ambient and elevated N plots in a temperate forest (Zak et al, 2017). However, given that the chemical fractionation method used in the present study (Wang et al, 1998) was unable to isolate pure specific compounds, whether or not the inhibition of SOC decomposition by N addition is different among specific biochemical compounds should be further investigated.…”

“…S2). Potential mechanims have been proposed to explain the N inhibition of SOC decomposition in aggregate-occluded and mineral-associated fractions (Neff et al, 2002;Cusack et al, 2011;Riggs et al, 2015;Zak et al, 2017). For example, N addition may alter decomposition of aggregate-occluded SOC through reducing carbon allocation to aggregate-binding agents such as fine roots and fungal hyphae (Li et al, 2015), and may inhibit decomposition of mineral-associcated SOC via acidifyiing soils and increasing mineral surface reactivity (Tian and Niu, 2015).…”

“…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.…”

“…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.…”

Humusica 2, article 18: Techno humus systems and global change – Greenhouse effect, soil and agriculture

“…Under European conditions with N depositions ranging from 20 to > 100 kg per ha and year, the question of increased N deposition and humus formation should also be addressed. N deposition have the potential to increase soil C storage (Zak et al, 2017). The explanation of organic matter accumulation in polluted soils was first searched in the impact of pollutants on microbial communities (Sterritt and Lester, 1980) but later studies showed that earthworm communities were strongly impacted by soil pollutants (Nahmani and Rossi, 2003), resulting in pronounced changes in soil structure and burying of litter.…”