Soil C and N changes under tillage and cropping systems in semi-arid Pacific Northwest agriculture
Introduction
An adequate amount of soil organic matter (SOM) is considered essential for long-term sustainable agriculture because declines generally decrease crop productivity (Allison, 1973). Changing SOM levels may alter the capacity for soil to act as a sink for atmospheric CO2 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 a significant impact on soil C and N content (De Jong and Kachanoski, 1988), and can substantially alter SOM sequestering pathways.
Semi-arid lands are especially susceptible to deterioration in SOM because of inherent low production and erratic yield. Greater use of crop residue for feed or fuel contributes to accelerated SOM loss. Fallowing is practiced extensively to increase water storage and stabilize crop yield, but it accelerates C and N loss from soil (Campbell et al., 1990; Rasmussen and Collins, 1991). Recent studies in sub-humid and semi-arid soils indicate a strong positive relationship between the amount of C incorporated into soil and the organic C content of that soil (Larson et al., 1972; Havlin et al., 1990; Rasmussen and Collins, 1991; Paustian et al., 1992). But even though crop residues have a beneficial effect on SOM, rising human population in semi-arid regions increases the use of crop residues for animal food and fuel and thus reduces residue return to soil. Modern tractors and farm machinery permit more intensive tillage of soil, which further accelerates in organic C (Rasmussen and Collins, 1991). Cropping practices must be defined in terms of their effect on rate of change in soil C and N in order to develop future strategies for maintaining soil quality.
Long-term experiments (LTEs) are the primary sources of information to determine the effects of cropping systems, soil management, fertilizer use, and residue utilization on changes in soil C and N over time (Leigh and Johnston, 1994). They are usually the only source of information to verify the accuracy of models used to identify soil capacity to sequester C and mitigate global climate change (Powlson et al., 1996). Recently, there have been both national and international efforts to utilize long-term experiments to determine agricultural sustainability (Barnett et al., 1995), define land-use effects on SOM (Paul et al., 1997), and test models of soil C-sequestration (Powlson et al., 1996).
Oregon State University maintains several LTEs at the Columbia Basin Agricultural Research Center near Pendleton, OR. Other LTEs were conducted at Moscow, ID; Lind and Pullman, Washington; and Moro, OR, between 1915 and 1945. In this paper, we summarize some of the results that define tillage and rotation effects on long-term C and N changes in semi-arid soils of the Pacific Northwest. We also address some of the deficiencies arising from the use of LTEs to evaluate C sequestration in soil.
Section snippets
Description of long-term experiments in the pacific northwest
Long-term research experiments were once located at Lind and Pullman in Washington, Moscow, ID, and Moro and Pendleton, OR. All have been terminated except for experiments at Pendleton. Research results from early long-term experiments conducted between 1920 to 1945 are included in this report to provide a perspective of early SOM change. Pendleton has six on-going long-term experiments, the earliest being established in 1931 and the latest in 1981 (Table 1).
Climatic, geographical and soil data
Crop rotation
Early studies show that soil C and N loss was always greater when crop rotations included fallow (Table 3). Average C loss was much greater at Pullman and Moscow than at Pendleton. Soils at the Pullman and Moscow sites have higher SOM, however, and the rate of SOM loss is linearly correlated with the amount initially present (Fig. 1). It appears difficult to prevent C loss from high SOM soils during the early years under cultivation when crop rotation includes fallow. High biological oxidation
Major factors controlling C and N change in soil
The major factors affecting organic C and N in semi-arid soils are the frequency of summer-fallow in crop rotations and the level of C input into soil through crop residue and added amendments. Trends for the Pacific Northwest agree very closely with those found for Canadian prairie soils (Campbell et al., 1990). Fallowing intensifies C loss from soil throughout the area, with indications that SOM cannot be maintained in rotations that include both fallow and conventional tillage. Decreasing
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