Abstract:
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. ResidueCinputs 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 Cinputs
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.
