Soil carbon and nitrogen accumulation with long-term no-till versus moldboard plowing overestimated with tilled-zone sampling depths

Abstract

Numerous investigators of tillage system impacts on soil organic carbon (OC) or total nitrogen (N) have limited their soil sampling to depths either at or just below the deepest tillage treatment in their experiments. This has resulted in an over-emphasis on OC and N changes in the near-surface zones and limited knowledge of crop and tillage system impacts below the maximum depth of soil disturbance by tillage implements. The objective of this study was to assess impacts of long-term (28 years) tillage and crop rotation on OC and N content and depth distribution together with bulk density and pH on a dark-colored Chalmers silty clay loam in Indiana. Soil samples were taken to 1 m depth in six depth increments from moldboard plow and no-till treatments in continuous corn and soybean–corn rotation. Rotation systems had little impact on the measured soil properties; OC content under continuous corn was not superior to the soybean–corn rotation in either no-till or moldboard plow systems. The increase in OC (on a mass per unit area basis) with no-till relative to moldboard plow averaged 23 t ha⁻¹ to a constant 30 cm sampling depth, but only 10 t ha⁻¹ to a constant 1.0 m sampling depth. Similarly, the increase in N with no-till was 1.9 t ha⁻¹ to a constant 30 cm sampling depth, but only 1.4 t ha⁻¹ to a constant 1.0 m sampling depth. Tillage treatments also had significant effects on soil bulk density and pH. Distribution of OC and N with soil depth differed dramatically under the different tillage systems. While no-till clearly resulted in more OC and N accumulation in the surface 15 cm than moldboard plow, the relative no-till advantage declined sharply with depth. Indeed, moldboard plowing resulted in substantially more OC and N, relative to no-till, in the 30–50 cm depth interval despite moldboard plowing consistently to less than a 25 cm depth. Our results suggest that conclusions about OC or N gains under long-term no-till are highly dependent on sampling depth and, therefore, tillage comparisons should be based on samples taken well beyond the deepest tillage depth.

Introduction

Soils can act as sinks (absorbers) or as sources (emitters) of greenhouse gases (Allmaras et al., 2000). Conventional tillage has frequently been associated with losses in soil organic carbon (OC), but less intensive tillage systems (such as no-till) have been effective in helping soils act as a carbon (C) sink. Minimizing soil disturbance reduces mineralization of organic matter (OM) and can result in larger storage of soil OC relative to conventional tillage (West and Post, 2002, Al-Kaisi and Yin, 2005).

Less intensive tillage with residue management practiced for extended periods of time has been shown to increase OC and N concentration and mass of the surface soil compared to the low steady state reached after many years of conventional tillage (Dick, 1983, Blevins, 1984, Eghball et al., 1994, Madari et al., 1998, Allmaras et al., 2000). Most previous studies (e.g. Balesdent et al., 1990, Karlen et al., 1994, Lal et al., 1994, Potter et al., 1998, West and Post, 2002) regarding tillage system effects on OC concentration and mass have focused on the “plow layer” (0–30 cm depth) and did not account for potential changes at deeper depths. According to Kern and Johnson (1993) the change from conventional tillage to no-till management sequesters the greatest amount of OC in the upper 15 cm of soil, and no significant amount below 15 cm. They concluded further – based on samples taken to a 30 cm depth – that no-till and conventional tillage had similar OC mass below 15 cm. In addition, guidelines developed for accounting of greenhouse gases in agricultural ecosystems generally provide estimates based on the upper 30 cm layer (Johnson et al., 1995, Eve et al., 2001). However, in studies of Angers et al. (1997) and Vandenbygaart et al., 2002, Vandenbygaart et al., 2003 it has been shown that although most soils under reduced tillage had more soil OC mass in the top 10 cm than under conventional tillage, the same reduced tillage systems had less OC mass at deeper depths (20–40 cm) than moldboard plow systems. Studies that have involved deeper sampling generally show no C sequestration advantage for conservation tillage, and gas exchange measurements also offer little support to the notion of a consistent soil C benefit from reduced tillage (Baker et al., 2007). The possible enrichment of soil C below the typical “plow layer” must be thoroughly investigated.

Although the purpose of many soil C studies is related to mitigating the impacts of global climate change, a better understanding of management-induced changes has benefits far beyond the current objectives of C sequestration projects. In sustainable agricultural production, the maintenance of soil OM quantity and quality is of key importance, and the replenishment of decreased levels of OM is slow and not easily achieved (Eswaran et al., 1993).

Tillage system influences on soil bulk density are variable, but important to quantify in evaluations of tillage influences on soil C content. Lal et al. (1994) found a decrease in bulk-density in no-till relative to conventional tillage, but some researchers have found no differences in soil bulk density near the soil surface (Hill and Cruse, 1985). Others, such as Gantzer and Blake (1978) for example, have found that bulk density in no-till is significantly higher near the surface when compared to plowing. Voorhees et al. (1978) hypothesized that the latter happens because when the soil remains untilled, it becomes denser over time due to the effect of consolidation. Even in temperate climates, the loosening effect of annual freezing and thawing cycles, wetting and drying, and soil organism activities are not enough to prevent this increase in soil bulk density. The relative soil bulk density changes also depend upon length of time in the no-till system and the traffic-induced compaction (Kladivko and Larney, 1989).

Bulk density is also directly influenced by the OM content of the soil. Its accurate measurement is critical for converting OC percentage on a weight basis to content per unit volume. Davidson et al. (1967) showed that bulk density is inversely related to soil OM. As the OM content increases, aggregation tends to increase thus increasing porosity and decreasing bulk density. No statement of tillage system influence on OC or N sequestration on a mass basis is accurate without a rigorous determination of bulk density.

The actual effect of different tillage practices on soil C storage is highly dependent on the crops produced in the field. Studdert and Echeverria (2000) concluded that the lowest soil OC content corresponded with soybean–soybean sequences and soil OC increased as soybean was not present or corn was present in the crop sequences they investigated. Differences in soil OC between crop rotations are more dependent on residue quantity than any other factor, although even that factor is conditioned by other factors (Batjes, 1996). Nitrogen fertilization, for instance, helps maintain soil OC with respect to the initial level, because it promotes plant growth and consequently results in more plant residue (Barber, 1979).

In the United States approximately 25% of the corn prior to 2002 followed corn in sequence (Power and Follett, 1987, NASS, 2006), and the vast majority of the remaining corn is grown in a 2-year rotation with soybean (Bullock, 1992, NASS, 2006). Relatively few studies have investigated soil OC and N differences between continuous corn and soybean–corn rotations after two decades or more.

The continuity of management practices is an important factor in C storage, since soil OC differences among treatments can take several years to develop. Soil response to different management practices varies depending in part upon the length of time the management practices have been in place (Potter et al., 1998, Omonode et al., 2006).

The objectives of the research were to (1) determine the long-term impact of conventional tillage (moldboard plowing) and conservation tillage (no-till) on soil OC and N mass and depth distribution in continuous corn and soybean–corn rotation to a 1 m depth, and (2) investigate the impact of the different tillage and rotation systems on soil bulk density and pH at alternate depth intervals.