Inclusion of cover crops (CCs) may be a potential strategy to boost no-till performance by improving soil physical properties. To assess this potential, we utilized a winter wheat (Triticum aestivum L.)-grain sorghum [Sorghum bicolor (L.) Moench] rotation, four N rates, and a hairy vetch (HV; Vicia villosa Roth) CC after wheat during the first rotation cycles, which was replaced in subsequent cycles with sunn hemp (SH; CrotaUria júncea L.) and late-maturing soybean [LMS; Glycine max (L.) Merr.] CCs in no-till on a silt loam. At the end of 15 yr, we studied the cumulative impacts of CCs on soil physical properties and assessed relationships between soil properties and soil organic C (SOC) concentration. Across N rates, SH reduced near-surface bulk density [\^^ by 4% and increased cumulative infiltration by three times relative to no-CC plots. Without N application, SH and LMS reduced Proctor maximum pj^, a parameter of soil compactibility, by 5%, indicating that soils under CCs may be less susceptible to compaction. Cover crops also increased mean weight diameter of aggregates (MWDA) by 80% in the 0-to 7.5-cm depth. The SOC concentration was 30% greater for SH and 20% greater for LMS than for no-CC plots in the 0-to 7.5-cm depth. The CC-induced increase in SOC concentration was negatively correlated with Proctor maximum pi and positively with MWDA and cumulative infiltration. Overall, addition of CCs to no-till systems improved soil physical properties, and the CC-induced change in SOC concentration was correlated with soil physical properties.Abbreviations: CC, cover crop; HV, hairy vetch; MWDA, mean weight diameter of aggregates; SH, sunn hemp; LMS, late-maturing soybean; SOC, soil organic carbon.

In agricultural systems, maintenance of soil organic matter (SOM) has long been recognized as a strategy to reduce soil degradation. No‐tillage and manure amendments are management practices that can increase SOM content and improve soil aggregation. We investigated the effects of 10‐yr of different tillage systems and N sources on soil aggregate‐size distribution and aggregate‐associated C and N. The study was a split‐plot design replicated four times. The main plot treatment was tillage (no‐tillage, NT; conventional tillage, CT) and the subplot treatment was N source (manure, M; NH4NO3 fertilizer, F). The experiment was established in 1990 on a moderately well‐drained Kennebec silt loam (Fine‐silty, mixed, superactive mesic Cumulic Hapludoll) with continuous corn (Zea mays L.). In 1999, soil samples were collected (0‐ to 5‐cm depth) from the field treatments and separated into four aggregate‐size classes (>2000, 250–2000, 53–250, and 20–53 μm) by wet sieving. Labile C and N content of all aggregate‐size fractions were measured using 28‐d laboratory incubations of intact and crushed aggregates. No‐tillage and M treatments significantly increased total C and N and the formation of macroaggregates. Conventional tillage in comparison with NT significantly reduced macroaggregates with a significant redistribution of aggregates into microaggregates. Aggregate protected labile C and N were significantly greater for macroaggregates, (>2000 and 250–2000 μm) than microaggregates (53–250 and 20–53 μm) and greater for M than F indicating physical protection of labile C within macroaggregates. No‐tillage and M a lone each significantly increased soil aggregation and aggregate‐associated C and N; however, NT and M together further improved soil aggregation and aggregate‐protected C and N.

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Five long-term tillage studies in Kansas were evaluated for changes in soil properties including soil organic carbon (SOC), water holding capacity (WHC), bulk density, and aggregate stability. The average length of time these studies have been conducted was 23 yr. Soil properties were characterized in three depth increments to 30 cm, yet changes due to tillage, N fertility, or crop rotation were found primarily in the upper 0-to 5-cm depth. Decreased tillage intensity, increased N fertilization, and crop rotations that included cereal crops had greater SOC in the 0-to 5-cm soil depth. Only one of five sites had greater WHC, which occurred in the 0-to 5-cm depth. Aggregate stability was highly correlated with SOC at all sites. No-tillage (NT) had greater bulk density, but values remained below that considered root limiting. Soil organic C levels can be modified by management that can improve aggregate stability, but greater SOC did not result in greater WHC for the majority of soils evaluated in this study. MATERIALS AND METHODS Five long-term study sites were selected across the state of Kansas as described in Tables 1 and 2, and located in Fig. 1.