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Title: DAYCENT simulated effects of land use and climate on county level N loss vectors in the USA

Author
item Del Grosso, Stephen - Steve
item PARTON, W - CSU, FORT COLLINS, CO
item OJIMA, D - CSU, FORT COLLINS, CO
item KEOUGH, C - CSU, FORT COLLINS, CO
item RILEY, T - CSU, FORT COLLINS, CO
item MOSIER, A - USDA-ARS, RETIRED

Submitted to: Complete Book
Publication Type: Book / Chapter
Publication Acceptance Date: 11/20/2007
Publication Date: 7/1/2008
Citation: Del Grosso, S.J., Parton, W.J., Ojima, D.S., Keough, C., Riley, T.H., Mosier, A.R. 2008. Daycent simulated effects of land use and climate on county level n loss vectors in the USA. Follett, R.F. and Hatfield, J.L. (eds). Nitrogen in the Environment: Sources, Problems, and Management 2nd Edition, p. 571-595. Elsevier Science Publishers, The Netherlands.

Interpretive Summary: We have described the DAYCENT ecosystem model and shown that annual nitrous oxide (N2O) emissions and nitrate (NO3) leaching can be reliably simulated for some managed and native systems. The model was used to explore how land use, precipitation, and soil texture impact total N losses and N gas emissions at the national scale using county level resolution simulations of cropped lands, grazed land, and native vegetation. Total N losses and the proportion of total losses due to NO3 leaching both tended to increase with N inputs. At the national scale, NO3 leaching was the major loss vector for both native and cropped/grazed systems because both N inputs and leaching are positively correlated with water inputs. However, leaching was responsible for less than half of total N losses for ~50% of the counties under native vegetation and ~15% of the counties for cropped/grazed systems. The counties where leaching did not make up the majority of N losses tended to be in the arid western half of the US. At the national scale, NH3 volatilization and NOx emissions made were responsible for more than 84% of N gas losses for grazed and native systems and about 58% of N gas losses for cropped systems. Similar to NO3 leaching, there was considerable variability, e.g., NH3 volatilization + NOx emissions were responsible for less than half of N gas emissions in ~3% of counties under native vegetation. Coarse textured soils tended to have both higher NO3 leaching losses and higher N gas losses than finer textured soils. In contrast to leaching and NOx losses, N2O and N2 emissions tended to increase as soils became finer textured. From a greenhouse gas perspective, fine textured soils are expected to emit more N2O, but from an N balance perspective, fine textured soils are expected to show smaller total N gas and leaching losses from the system. We conclude that N losses from soils are strongly dependent on land management but that generalizations based solely on soil N and water inputs are likely to be limited because soil texture, soil C levels, and plant demand for nutrients are also important.

Technical Abstract: We describe the nitrogen (N) gas (NH3, NOx, N2O, N2) emission and NO3 leaching submodels used in the DAYCENT ecosystem model and demonstrate the ability of DAYCENT to simulate observed N2O emission and NO3 leaching rates for various sites representing different climate regimes, soil types, and land uses. DAYCENT simulated seven major crops, grazing lands, and potential native vegetation at the county level for the USA. At the national scale, NO3 leaching was the major loss vector, accounting for 86%, 66% and 56% of total N losses for cropped soils, grazed lands, and native vegetation, respectively. NH3 volatilization + NOx emissions made up the majority of national N gas losses, accounting for 58%, 89%, and 86% of N gas losses from cropped soils, grazed lands, and native vegetation, respectively. However, there was considerable spatial variability in the N loss vectors, with leaching accounting for less than 20% of total N losses and NOx + NH3 emissions accounting for less than 50% of N gas losses in some counties. Land use area weighted mean annual N losses were 43.9 (std. dev. = 26.8) and 12.3 (std. dev. = 22.2) kg N ha-1 for cropped/grazed and native systems, respectively. Area weighted mean annual N gas losses were 11.8 (std. dev. = 4.8) and 5.4 (std. dev. = 2.1) kg N ha-1 for cropped/grazed and native systems, respectively. Total N losses and NO3 leaching tended to increase as N inputs and precipitation increased, and as soils became coarser textured. Total N gas losses also increased with N inputs and as soils became coarser textured, but N2O and N2 made up a larger portion of N gas losses as soils became finer textured and as precipitation increased.