Phosphorus budgets of intensively managed row crops at a long-term agroecosystem research site in the upper US Midwest

Abstract

Phosphorus (P) budgets for cropping systems provide insights for keeping soil P at optimal levels for crops while avoiding excess inputs. We quantified 12 years of P inputs (fertilizer and atmospheric deposition) and outputs (harvest and leaching losses) for replicated maize (Zea mays L.)-soybean (Glycine max L.)-wheat (Triticum aestivum) crop rotations under conventional, no-till, reduced input, and biologically based (organic without compost or manure) management systems at the Kellogg Biological Station LTAR site in southwest Michigan. Conventional, no-till, and reduced input systems were fertilized between 13 and 50 kg P ha^-1 depending on year. Soil test phosphorus (STP) was measured at 0- to 25-cm depth every autumn. Leached P was measured as dissolved P in the soil solution sampled beneath the rooting depth and combined with modeled percolation. Fertilization and harvest were the predominant P fluxes in the fertilized systems, whereas only harvest dominated P flux in the unfertilized organic system. Leaching losses were minor terms in the budgets, but leachate concentrations were nevertheless close to the range of concern for downstream eutrophication. Over the 12-year study period, the organic system exhibited a negative P balance (-82.0 kg P ha^-1), coinciding with suboptimal STP levels, suggesting a need for P supplementation. In contrast, the fertilized systems showed positive P balances (mean: 70.1 kg P ha^-1) with STP levels well above agronomic optima. Results underscore the importance of tailored P management strategies to sustain crop productivity while mitigating environmental impacts.

FIGURE 1

Annual phosphorus removal by crop harvest in the four cropping systems. The maize‐soybean‐wheat rotation began in 2008, with four complete rotations over the 12 years. Maize, soybean, and wheat years are represented by solid, stippled, and hatched columns, respectively. For wheat, harvest P removal is partitioned into grain (diagonal) and stover (crosshatch) in the conventional and no‐till systems. Bars indicate standard errors of the mean for five replicate plots. When cropping systems share a similar letter, the means grouped across years are not significantly different as determined by the Tukey honest significant difference (HSD) post hoc test (α = 0.05).

FIGURE 2

Annual soil test phosphorus (STP) concentrations in the upper 25 cm in the four cropping systems. The maize‐soybean‐wheat rotation began in 2008, with four complete rotations over the 12 years. Maize, soybean, and wheat years are represented by solid, stippled, and hatched columns, respectively. The inset shows which system means grouped across years are significantly different (*) in pairwise comparisons as determined by the Tukey honest significant difference (HSD) post hoc test (α = 0.05; NS, not significantly different). The horizontal line indicates the critical lower soil P threshold for maize and soybean production (68 kg P ha⁻¹, equivalent to 15.0 mg kg⁻¹).

FIGURE 3

Cumulative phosphorus balance (inputs minus outputs) for the four management systems. In the biologically based (organic) system, output and balance lines are virtually superimposed.