Soil C and N changes under tillage and cropping systems
in semi-arid Paci®c Northwest agriculture
P.E. Rasmussen
a,*
, S.L. Albrecht
a
, R.W. Smiley
b
a
USDA, Agricultural Research Service, Columbia Plateau Conservation Research Center, PO Box 370, Pendleton,
OR 97801 (48037 Tubbs Ranch Road, Adams, OR 97810), USA
b
Columbia Basin Agricultural Research Center, Oregon State University Agricultural Experiment Station, PO Box 370,
Pendleton OR 97801, USA
Abstract
Soils in semi-arid regions are highly susceptible to soil organic matter (SOM) loss when cultivated because of erratic yield,
removal of crop residue for feed or fuel, uncontrolled soil erosion, and frequent fallowing to increase water storage. It is
important to quantify the effect of each factor to be able to identify agoecosystems that are sustainable and recognize the
management practices that best sequester C in soil. We identi®ed changes in SOM in long-term experiments, some dating from
the early 1900s, by evaluating tillage and crop rotation effects at several locations in semi-arid regions of the US Paci®c
Northwest. The major factors in¯uencing changes in organic C and N were the frequency of summer-fallow and the amount of
C input by crop residue. Soil erosion was low in long-term studies, but even limited soil loss can have a substantial impact on
C and N levels if allowed over many years. Yearly crop production is recommended because any cropping system that
included summer-fallow lost SOM over time without large applications of manure. We conclude that most of the SOM loss
was due to high biological oxidation and absence of C input during the fallow year rather than resulting from erosion.
Decreasing tillage intensity reduced SOM loss, but the effect was not as dramatic as eliminating summer-fallow. Crop
management practices such as N fertilization increased residue production and improved C and N levels in soil. SOM can be
maintained or increased in most semi-arid soils if they are cropped every year, crop residues are returned to soil, and erosion is
kept to a minimum. SOM loss may be more intense in the Paci®c Northwest because fallowing keeps the soil moist during the
summer months when it would normally be dry. Our experiments identify two primary de®ciencies of long-term studies to
measure C sequestering capability: (1) soil C loss can be partitioned between erosion and biological oxidation only by
estimation, and (2) C changes occurring below 30 cm in grassland soils cannot be quanti®ed in many instances because
samples were not collected. # 1998 Elsevier Science B.V. All rights reserved.
Keywords: Decomposition; Fallow; Fertilizer; Long-term experiments; Manure; Residue; Soil moisture; Soil organic matter; Wheat
1. Introduction
An adequate amount of soil organic matter (SOM)
is considered essential for long-term sustainable agri-
culture because declines generally decrease crop pro-
ductivity (Allison, 1973). Changing SOM levels may
alter the capacity for soil to act as a sink for atmos-
pheric CO
2
and impact global climate change (Esser,
1990; Rounsevell and Loveland, 1994). Cropping
intensity, tillage, residue input, and erosion all affect
SOM. Both biological oxidation and soil erosion have
Soil & Tillage Research 47 (1998) 197±205
*Corresponding author. Tel.: 541 278 4380; fax: 541 278 4372;
e-mail: paul.rasmussen@orst.edu
0167-1987/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved.
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