Amounts and Forms of Dissolved Phosphorus Lost with Surface Runoff as Affected by Phosphorus Management and Soil Conservation Practices
Surface runoff accounts for the majority of phosphorus (P) from Iowa fields delivered to streams. Important fractions of runoff P are dissolved reactive P (orthophosphate P) and particulate (sediment-bound) P. Dissolved P is readily available to aquatic organisms and a large proportion of particulate P be comes available over a period of time depending on properties of the receiving water body. Recent surveys of Iowa streams and in the Lake Erie watershed suggest the amount of dissolved P loss from fields and its impact on water quality is greater than often assumed. Iowa research has shown higher orthophosphate P loss with fertilizer than with manure, and some conservation practices that reduce erosion and particulate P loss may not affect or may even increase dissolved P loss.
The goal of this project is to study dissolved P in runoff for a wide range of soil P levels, fertilizer and manure P management practices, and soil conservation practices. Specific objectives are to determine the amount of runoff dissolved P not measured by the commonly used dissolved reactive P method, and study how amounts of different dissolved P forms in runoff can be estimated by soil-test P methods recommended for crops, water-extractable soil P, and an index of soil P saturation.
About 900 soil and runoff samples from other ongoing and recently completed experiments will be analyzed. These samples represent field experiments managed with natural or simulated rainfall that included different soil-test P levels, crops, fertilizer and manure P placement methods and times of application, tillage systems, soil or manure amendments, corn harvesting systems, and the conservation practices cover crops and buffer grass strips.
The study demonstrated that the commonly used measurement of dissolved-reactive phosphorus in runoff often underestimates total dissolved P and bioavailable P losses. Across all runoff samples analyzed, dissolved reactive P, total dissolved P and bioavailable P were 61, 72 and 72 % of the total P, respectively. Therefore, total dissolved P was greater than dissolved reactive P and similar to bioavailable P, and 16% of the total dissolved or bioavailable P was wrongly considered particulate P. The dissolved and bioavailable P forms in runoff not measured by the dissolved reactive P measurement also are very effective at encouraging rapid eutrophication of surface water resources.
The several P, soil and crop management practices for corn-soybean rotations included in the study influenced in different ways the proportion of the total runoff P comprised by dissolved and bioavailable P forms, however. Increasing soil-test P, soil P saturation or the P application rate greatly increased the proportions of dissolved reactive and total dissolved P in runoff, but did not affect the dissolved reactive P underestimation of the total dissolved P.
No-till management increased the proportion of the total runoff P loss comprised by dissolved and bioavailable P compared with tillage. These were higher in the corn year (planted on soybean residue) than in the soybean year (planted on corn residue) with both tillage systems, and the dissolved reactive P underestimation of the total dissolved P loss was slightly larger with tillage than with no-till mainly in the soybean year (planted on corn residue).
The P source effects on runoff P varied with the tillage system. With tillage, the proportion of the total P comprised by dissolved and bioavailable P were the smallest for poultry manure and the largest for fertilizer and swine manure, but with no-till were the highest for fertilizer and did not differ for the two manures. The dissolved reactive P underestimation of dissolved P loss much smaller for fertilizer and liquid swine manure than for solid poultry manure.
Of alum and gypsum amendments, only alum significantly reduced the proportion of the total runoff P loss comprised by dissolved and bioavailable P. It also increased the underestimation of dissolved P by the dissolved reactive P measurement.
Use of a winter cereal rye cover crop reduced dissolved and total runoff P loss with tillage and no-till management, but the reductions were much larger with tillage. The cover crop did not affect the dissolved reactive P underestimation of dissolved P loss. In contrast, use of filter strips increased dissolved reactive P and dissolved P concentrations in surface runoff, but slightly decreased the dissolved reactive P underestimation of the total dissolved P concentration.
More detailed findings are available in the comprehensive Final Project Report.