Soil carbon and nitrogen changes as influenced by tillage and cropping systems in some Iowa soils
Introduction
Soil organic C (SOC) and total N (TN) contents play a crucial role in sustaining soil quality, crop production, and environmental quality (Bauer and Black, 1994, Doran and Parkin, 1994, Robinson et al., 1994) due to their effects on soil physical, chemical, and biological properties, such as soil water retention, nutrient cycling, gas flux, and plant root growth (Sainju and Kalisz, 1990, Sainju and Good, 1993). Soil, as an open system, can be a net source of CO2 released to the atmosphere due to elevated SOC mineralization as a result of disruptive agricultural practices. On the other hand, soil can function as a net sink for sequestering atmospheric CO2 under appropriate soil and crop management, and thus reducing atmospheric CO2 (Paustian et al., 1992, Lal et al., 1995).
Plant biomass is a source of C and N, which can replenish SOC and TN. Changes in soil conditions (e.g., temperature, moisture, O2, pH, and nutrient availability) can alter the decomposition rate of plant biomass and the mineralization rate of soil organic matter (Broadbent et al., 1964, Kowalenko et al., 1978, Clark and Gilmour, 1983). Therefore, SOC and TN can be enriched via appropriate soil and crop management practices that either increase organic matter input to the soil, decrease the mineralization rate of soil organic matter, or both (Paustian et al., 2000, Follett, 2001).
It has been well documented that soil can be managed to increase SOC and TN storage from a long-term (>10 years) perspective by implementing conservation soil and crop management practices such as conservation tillage (Havlin et al., 1990, Franzluebbers et al., 1995, Halvorson et al., 2002) and crop rotations (Robinson et al., 1996). However, short-term (≤10 years) management effects on soil C and N dynamics are complex and often variable. After analyzing a large global data set, West and Post (2002) concluded that soil C sequestration was generally increased by no-tillage practices, but had a delayed response, with peaks in years 5–10. This finding agreed with the results reported by Franzluebbers and Arshad (1996), that there may be little to no detectable increase in SOC in the first 2–5 years, but a large increase 5–10 years after switching to conservation tillage. In a study on short-term crop rotation effects on SOC, Campbell et al. (2000) found that measurable gain in SOC could be observed in 6 years or less when weather conditions were favorable.
Conservation tillage systems such as no-tillage, strip-tillage, and chisel plowing have been increasingly used in the Midwest during the past decade due to their profitability and environmental advantages over moldboard plowing. For example, no-tillage systems in the Midwest were used in over 22% of all cropland area in 2002 according to the 2002 National crop residue management survey conducted by the Conservation Technology Information Center (unpublished data), which almost doubled that in 1992. Deep ripping is an effective and popular tool that can be used to overcome soil compaction in the Midwest. Although, deep ripping is not a conservation tillage system, it still results in less soil disturbance than moldboard plowing. However, there have been few studies that quantify these main tillage alternatives with different tillage intensities on soil C and N changes over time compared with moldboard plowing in the Midwest soils where a corn (Zea mays L.)–soybean (Glycine max (L.) Merr.) rotation is the primary cropping system.
Conversion of cropland particularly marginal land to pasture has the greatest potential for increasing C sequestration in the soil (Lal et al., 1998). Corn–soybean–alfalfa (Medicago sativa L.) rotation is an effective and primary crop rotation system that has been used in the Midwest to improve soil quality and productivity. Smooth bromegrass (Bromus inermis Leyss.), and switchgrass (Panicum virgatum L.) are the two main perennial grass species that have been widely used for grazing in the Midwest. There is very little information on whether these perennial grass cropping systems are superior to a corn–soybean–alfalfa rotation in improving soil C and N storage under the Midwest production environment.
The effects of soil and crop management practices on SOC and TN dynamics, in part, depend on soil properties and environmental factors, such as soil texture, clay mineralogy, topography, and climate (Janssen, 1984, Bohn et al., 1985, Campbell et al., 1999). Therefore, an understanding of possible differential effects of management practices on SOC and TN dynamics of different soil types and under different production areas is essential in developing best management practices and prediction tools for SOC and TN management. The objectives of this study were to (1) examine short-term SOC and TN responses to various tillage alternatives in the CNW soil association under a corn–soybean rotation; (2) evaluate whether tillage (no-tillage and chisel plowing) effects on SOC and TN differ on different soil associations under a corn–soybean rotation; and (3) assess the effects of perennial smooth bromegrass and switchgrass cropping systems on SOC and TN contents relative to a corn–soybean–alfalfa rotation under no-tillage management.
Section snippets
Materials and methods
This study consisting of three different experiments was designed to evaluate the short-term effects of tillage and cropping systems on SOC and TN contents in various soil associations across Iowa. Location and basic soil information of each experiment are presented in Table 1.
2-tillage experiment
When data of the five soil associations were analyzed separately in this experiment, the difference in either SOC or TN content in 2000 between no-tillage and chisel plowing was not significant in any of the soil associations (Table 3). However, all the soil associations showed an increasing trend in both SOC and TN contents with no-tillage compared with chisel plowing at the 0–15 cm soil depth. Statistical insignificance in SOC and TN contents in response to tillage systems at the 0–15 cm soil
Conclusions
Short-term tillage effects on SOC and TN contents occurred primarily in the 0–15 cm soil depth. Tillage effects on SOC and TN did not vary significantly with soil association. No-tillage resulted in greater SOC and TN contents at the 0–15 cm soil depth than chisel plowing at the end of 7 years of tillage practices averaged across the CNW, GPS, KFC, M, and OMT soil associations in a corn–soybean rotation. The increase in SOC and TN with no-tillage was not related to SOC and TN stratification in
Acknowledgments
We thank Dr. Antonio Mallarino, Department of Agronomy, Iowa State University and Dr. Mark Hanna, Department of Agricultural and Biosystem Engineering, Iowa State University for their contributions to this study.
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