Cover crops can mitigate no-tillage-induced labile phosphorus stratification

Abstract

Minimal- and no-tillage systems are effective at reducing soil erosion, increasing soil organic matter, and reducing nutrient losses. However, extended periods of reduced tillage can stratify phosphorus (P) in surface soil layers, which can have negative implications for both water quality and crop nutrition. We measured soil P stratification in a long-term (12-year) experiment under maize (Zea mays)–soybean (Glycine max) rotations that combine two tillage practices (chisel-plow, no-tillage) with/without a winter cover crop (Secale cereale or cereal rye). Our objectives were twofold: (1) to quantify soil P stratification in this 2 × 2 factorial experiment using a common P stratification index (P_strat) on six soil P measurements—water-extractable (H₂O-P), microbial biomass (MBP), anion exchange resin extractable phosphorus (AER-P), soil test Mehlich-3 P (STP), bicarbonate extractable phosphorus, and total P. We calculated P_strat as mean P concentrations at 0–5 cm divided by 5–25 cm depths and (2) to correlate P_strat to crop P uptake and yield to answer the question—Does P stratification limit crop growth? The P_strat ranged from 1.3 to 87.5 across all soil P measurements, but H₂O-P was most stratified (mean H₂O-P_strat = 22.8). No-tillage increased P_strat by 8%–584% compared to chisel plowing, and cover crops (with tillage) increased P_strat by 8%–269% compared to winter fallow with tillage. In an unexpected antagonistic interaction, however, adding a cover crop to no-tilled soils reduced P_strat by 23%–72% compared to no-till. Interestingly, soybean P uptake and yield were positively related to a few P_strat measures (r > 0.48); but maize P uptake was negatively related to stratification of STP (r < −0.46). We confirmed that long-term no-tillage, and even cover crops alone, can stratify soil P. When combined, however, cereal rye as a winter cover crop can alleviate no-till-caused soil P stratification, adding yet another benefit to using this winter-hardy cover crop in US Midwest cropping systems.