POTENTIAL FOR MINERALIZATION OF SOIL ORGANIC MATTER TO SUPPLY CROP NITROGEN, INFLUENCE CROP YIELD AND IMPROVE NITROGEN RECOMMENDATIONS IN WISCONSIN SANDY SOILS

Abstract

Wisconsin is an important state for agricultural food production. In particular, the Wisconsin Central Sands (WCS) region, has a large number of acres in vegetable production. The soils in this region are coarse textured and often deep, with low nutrient status and water holding capacity. This necessitates inputs of fertilizer and irrigation water for crop production to be profitable. Even when producers follow best management practices laid out by University of Wisconsin Extension, environmental factors can lead to nutrients leaching past the rooting zone of crops. Large rainfall or irrigation events can carry nutrients, in particular nitrogen (N) to groundwater faster than crops can take them up. Nitrogen lost from these agroecological systems can travel far from the point of application and cause a cascade of environmental and health concerns once in groundwater drinking supplies, and surface water, particularly in the Gulf of Mexico. Sandy soils in temperate regions of the US often have low, but variable, amounts of soil organic matter (SOM). Current N application recommendations for corn in WI do not consider SOM content although SOM can influence N mineralization and availability. For all other crops, N application rates are recommended for ranges of SOM. The potential for SOM to mineralize and supply N for crop needs should be further investigated. Particularly in sandy soils in central Wisconsin that have been drained by ditches to historically promote agriculture in the region. The objective of this study was to quantify the effect of SOM on N mineralization, crop N uptake, and yield. Soil and plant samples were collected from an agricultural field in vegetable production in the WCS containing a gradient of SOM content, ranging from 4% (low) to 9% (medium) and 15% (high) across the field. Soil plant available N and potentially mineralizable N (PMN) were measured biweekly during the 2019 and 2020 growing seasons to observe changes in soil N pools across time. Aboveground plant N and corn grain yield were estimated in 2019. We found that medium SOM plots had the largest early to mid-season soil plant available N and plant uptake in 2019, which coincided with periods of rapid plant growth, but this did not result in significantly higher grain yield (medium: 11,652 kg ha⁻¹; low: 8,897 kg ha⁻¹; high: 7,753 kg ha⁻¹). Potentially mineralizable N concentrations were consistently larger in medium and high SOM plots than in low SOM plots throughout both growing seasons. In 2019, medium SOM plots had the most PMN on a field-scale on nearly every sampling date, particularly during May through July when crop N demand was highest. In 2020, under a pea–millet rotation, PMN was similarly larger in medium and high SOM plots throughout the growing season, although plant uptake could not be quantified in kg ha⁻¹ due to missing plant density data. High SOM plots in both years often had more PMN later in the season, when crop uptake slowed, highlighting the potential for N loss via leaching. These results indicate that SOM influences both the magnitude and timing of N availability, with medium SOM providing the closest alignment of soil N supply with crop demand in sandy soils of the WCS. This research highlights how SOM gradients, including legacy SOM from past soil formation, can affect the synchronization of soil N supply with crop need in sandy agricultural soils. A better accounting of SOM in fertilizer recommendations could improve N use efficiency (NUE), while maintaining yield and reducing environmental degradation due to N loss in the WCS and other similar agroecosystems.