, Volume 107, Issue 1-3, pp 207-225
Date: 17 Nov 2010

Carbon balances in US croplands during the last two decades of the twentieth century

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Carbon (C) added to soil as organic matter in crop residues and carbon emitted to the atmosphere as CO2 in soil respiration are key determinants of the C balance in cropland ecosystems. We used complete and comprehensive county-level yields and area data to estimate and analyze the spatial and temporal variability of regional and national scale residue C inputs, net primary productivity (NPP), and C stocks in US croplands from 1982 to 1997. Annual residue C inputs were highest in the North Central and Central and Northern Plains regions that comprise ~70% of US cropland. Average residue C inputs ranged from 1.8 (Delta States) to 3.0 (North Central region) Mg C ha−1 year−1, and average NPP ranged from 3.1 (Delta States) to 5.4 (Far West region) Mg C ha−1 year−1. Residue C inputs tended to be inversely proportional to the mean growing season temperature. A quadratic relationship incorporating the growing season mean temperature and total precipitation closely predicted the variation in residue C inputs in the North Central region and Central and Northern Plains. We analyzed the soil C balance using the crop residue database and the Introductory Carbon Balance regional Model (ICBMr). Soil C stocks (0–20 cm) on permanent cropland ranged between 3.07 and 3.1 Pg during the study period, with an average increase of ~4 Tg C year−1, during the 1990s. Interannual variability in soil C stocks ranged from 0 to 20 Tg C (across a mean C stock of 3.08 ± 0.01 Pg) during the study period; interannual variability in residue C inputs varied between 1 and 43 Tg C (across a mean input of 220 ± 19 Tg). Such interannual variation has implications for national estimates of CO2 emissions from cropland soils needed for implementation of greenhouse gas (GHG) mitigation strategies involving agriculture.