Soil C and N changes under tillage and cropping systems in semi-arid Pacific Northwest agriculture

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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 identified 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 Pacific Northwest. The major factors influencing 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 Pacific Northwest because fallowing keeps the soil moist during the summer months when it would normally be dry. Our experiments identify two primary deficiencies 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 quantified in many instances because samples were not collected.

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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