Impact of rotation, cover crop and manure inputs on productivity, soil organic matter fractions and soil nitrogen in irrigated Michigan potato cropping systems

Michigan growers have begun to integrate winter rye (Secale cereale) cover crops and manure amendments into potato (Solanum tuberosum) cropping systems to decrease soil erosion and nutrient losses and to improve soil organic matter (SOM). Information about cover crop biomass accumulation and effects within the constraints of a Michigan potato cropping system is limited. Objectives of these studies were to 1) quantify winter cover crop biomass accumulation on commercial potato farms and in controlled potato cropping field studies, 2) understand effect of maturity on important cover crop biochemical qualities, 3) compare productivity, soil organic matter fractions and nutrient pools in field studies where potatoes were rotated with wheat, corn or snap beans with and without manure amendments. Mid-April above-ground rye biomass on commercial farms ranged from 10 to 2600 kg ha-1 over two years and averaged about 900 kg ha-1. Root biomass was typically 2 to 5 times greater than shoot biomass in these fields. Most commercial Michigan potato farms are achieving sufficient winter rye cover crop maturity and biomass for weed suppression and erosion and nutrient loss reduction, but are probably not generating sufficient biomass to restore SOM without additional inputs. A field experiment demonstrated that mid-September to mid-October cover crop planting can increase winter rye and rye-hairy vetch (Vicia villosa L. Roth) biomass compared with biomass measured in commercial fields. An additional 4 to 6 weeks of fall growth can double above-ground rye biomass in the spring. During early spring growth (from 416 to 532 GDD) above ground biomass was accumulated at 144, 232 and 39 kg ha -1 per 10 GDD for rye, rye-hairy vetch and hairy vetch cover crops, respectively. For the 532 to 962 GDD growing period, above-ground biomass accumulated at 97 and 218 kg ha-1 per 10 GDD (across all cover crops) for zero and 30 kg N ha-1 treatments. Below-ground biomass changes for 6 cover crop-soil inorganic N content combinations were smaller and more variable. Above-ground cover crop tissue organic matter (OM), neutral detergent fiber (NDF) and acid detergent lignin (ADL) content were significantly affected by species and soil N fertility on some sampling dates. Both NDF and ADL fractions increased with maturity across species and N treatment. Early June soil macro-POM-C and -N pools reflected biomass and quality of cover crops, but these fractions did not persist through the growing season. By late October, Potato-Wheat/Clover rotation plots had higher macro-POM fractions than Potato-Snap bean rotations, regardless of winter cover crop, despite having low macro-POM fractions in early June. Across 3 sampling dates at both locations, Potato-Wheat/Clover rotation plots were consistently among the treatments with the highest POM-C and -N fractions and the lowest POM C:N ratios. Potato-Wheat/Clover rotation plots and all non-potato phase plots had the greatest inorganic N availability. Residual inorganic N after crop removal was generally higher for potato phase plots than for non-potato phase plots to a depth of 51 cm. In a production trial, cover crop treatment did not affect total or US No. 1 tuber yields consistently, but 5.6 Mg ha -1 poultry manure amendment with fertilizer reduction consistently increased US No. 1 tuber yield by an average of 17.6%. Rye cover crop, with and without poultry manure, were the only treatment combinations to produce positive marginal revenues compared with an unamended bare control treatment.