Net Nitrogen Mineralization in Natural Ecosystems across the Conterminous US

Abstract

Nitrogen is the primary nutrient limiting ecosystem productivity over most of the US. Although soil nitrogen content is important, knowledge about its spatial extent at the continental scale is limited. The objective of this study was to estimate net nitrogen mineralization for the conterminous US (CONUS) using an empirical modeling approach by scaling up site level measurements. Net nitrogen mineralization and total soil nitrogen data across the CONUS were obtained from three different ecosystems: low elevation forests, high elevation forests, and grasslands. Equations to predict net nitrogen mineralization were developed through stepwise linear regression using total Kjeldahl nitrogen, air temperature, precipitation, and nitrogen deposition as predictor variables for four categories: low elevation high temperature forests (coefficient of determination, R 2 = 0.83), low elevation low temperature forests (R 2 = 0.74), high elevation forests (R 2 = 0.80), and grasslands (R 2 = 0.88). A map of net nitrogen mineralization was developed in GIS using these equations and national-scale databases for the CONUS. The result shows that net nitrogen mineralization varies widely across the US. Grasslands were predicted to have the lowest net nitrogen mineralization, while low elevation forests in the east had the highest. Mean values were 14.3 kg•ha −1 •yr −1 for grasslands, 22.6 kg•ha −1 •yr −1 for high elevation forests, 58 kg•ha −1 •yr −1 for low elevation low temperature forests, and 82.9 kg•ha −1 •yr −1 for low elevation high temperature forests. This continental scale estimation of net nitrogen mineralization provides a means of comparing net nitrogen mineralization across regions, and the databases developed from this study are useful for accounting for nitrogen limitations in large scale ecosystem modeling.

Climate Change Predicted to Negatively Influence Surface Soil Organic Matter of Dryland Cropping Systems in the Inland Pacific Northwest, USA

“…Variability in N mineralization has been reported to be primarily correlated with MAT, MAP, as well as total N and N deposition (Chapman et al, 2013). In agreement with our results, Colman and Schimel (2013) concluded that the direct drivers of N mineralization were soil C and N as well as clay content and that precipitation drives N mineralization indirectly through its influence on soil C and N content.…”

Ambient nitrogen deposition drives plant‐diversity decline by nitrogen accumulation in a closed grassland ecosystem

“…This would, in turn, contribute to N mineralization by increases in substrates. It has been predicted that the mean value of net N mineralization in natural grasslands was 14.3 kg N ha −1 yr −1 (Chapman et al, 2013). This Nmineralization rate, as well as relatively low rainfall in the grassland, may lead to inorganic N accumulation in soil (Figure 4).…”

Winter runoff events pose an unquantified continental-scale risk of high wintertime nutrient export

“…To create a map of total nitrogen pools across the contiguous US, we combined historical wet inorganic nitrogen deposition [78], fertilizer and manure nitrogen application [77], and topsoil nitrogen [75,83] maps. For total soil nitrogen pools, we used total Kjeldahl nitrogen (0-50 cm; kg N ha −1 ) in raster (1 km) format [75,83] (figure S1(a)).…”

“…As an example of the potential importance of these events, we focus on the impact of one winter flood-generating event, rain-on-snow, in the contiguous US. We assess the potential for rain-on-snow to impact water quality by collecting and combining datasets on topsoil nitrogen and phosphorus pools [75,76], annual nutrient inputs from fertilizer [77] and atmospheric deposition [78], and the historical daily frequency of heavy rainfall on snow-cover defined as a large rain-on-snow event [79]. We use these continental-scale data to assess the spatial cooccurrence of rain-on-snow and nutrient-rich areas in the US.…”