A gronomy J our n al • Volume 110 , I ssue 1 • 2 018 1 T he goal of an N recommendation system is to accurately estimate the gap between the N provided by the soil and the N required by the plant. Accurately estimating this gap depends on the ability of the recommendation system to accurately estimate fi eld or subfi eld specifi c economically optimal nitrogen rates (EONR). Current recommendation systems are not as accurate as needed to provide consistently reliable estimates of N needs across years at the fi eld or subfi eld scale. Uncontrollable factors like temperature, rainfall timing, intensity and amount, and interactions of temperature and rainfall with factors such as N source, timing and placement, plant genetics, and soil characteristics combine to make N rate recommendations for an individual fi eld or rates for subfi elds a process guided as much by science as by the best professional judgement of farmers and farm advisors.Substantial evidence has accumulated that EONRs can vary widely across fi elds, within fi elds and over years in the same fi eld for a wide range of crops and geographies. Examples ABSTRACTNitrogen fi xation by the Haber-Bosch process has more than doubled the amount of fi xed N on Earth, signifi cantly infl uencing the global N cycle. Much of this fi xed N is made into N fertilizer that is used to produce nearly half of the world's food. Too much of the N fertilizer pollutes air and water when it is lost from agroecosystems through volatilization, denitrifi cation, leaching, and runoff . Most of the N fertilizer used in the United States is applied to corn (Zea mays L.), and the profi tability and environmental footprint of corn production is directly tied to N fertilizer applications. Accurately predicting the amount of N needed by corn, however, has proven to be challenging because of the eff ects of rainfall, temperature, and interactions with soil properties on the N cycle. For this reason, improving N recommendations is critical for profi table corn production and for reducing N losses to the environment. Th e objectives of this paper were to review current methods for estimating N needs of corn by: (i) reviewing fundamental background information about how N recommendations are created; (ii) evaluating the performance, strengths, and limitations of systems and tools used for making N fertilizer recommendations; (iii) discussing how adaptive management principles and methods can improve recommendations; and (iv) providing a framework for improving N fertilizer rate recommendations.

Rising concerns about greenhouse gases, increased fuel prices, and the potential for new high value agricultural products have raised interest in the use of maize stover for bioenergy production. However, residue harvest must be weighed against potential negative impacts on soil quality. This study, conducted in Chazy, NY, evaluated the long‐term effects of 32 yr of maize (Zea mays L.) stover harvest vs. stover return on soil quality in the surface layer (5–66 mm) under plow till (PT) and no‐till (NT) systems on a Raynham silt loam (coarse‐silty, mixed, active, nonacid, mesic Aeric Epiaquept) using physical, chemical, and biological soil properties as soil quality indicators. Twenty‐five soil properties were measured, including standard chemical soil tests, aggregate stability (WSA), bulk density, (ρb) penetration resistance (PR), saturated hydraulic conductivity (Ks), infiltrability (Infilt), several porosity indicators (aeration pores(PO > 1000), soil water potential = Ψ > −0.36 kPa; air‐filled pores at field capacity (PO > 30), Ψ > −10kPa; available water capacity (AWC), −1500 < Ψ < −10 kPa), total organic matter (OM), parasitic (Nemparasitic) and beneficial nematode (Nem beneficial) populations, decomposition rate (Decomp), potentially mineralizable N (PMN) and easily extractable (EEG) and total glomalin (TG). Only eight indicators were adversely affected by stover harvest, and most of these effects were significant only under NT. Almost all indicators affected by stover removal were affected equally or more adversely by tillage. A total of 15 indicators were adversely affected by tillage. Results of this study suggest that, on a silt loam soil in a temperate climate, long‐term stover harvest had lower adverse impacts on soil quality than long‐term tillage. Stover harvest appears to be sustainable when practiced under NT management.

Understanding the response of soil quality indicators to changes in management practices is essential for sustainable land management. Soil quality indicators were measured for 2 years under established experiments with varying management histories and durations at four locations in New York State. The Willsboro (clay loam) and Aurora (silt loam) experiments were established in 1992, comparing no-till (NT) to plow-till (PT) management under corn (Zea mays L.)-soybean (Glycine max L.) rotation. The Chazy (silt loam) trial was established in 1973 as a factorial experiment comparing NT versus PT and the crop harvesting method (corn silage versus corn grain). The Geneva (silt loam) experiment was established in 2003 with vegetable rotations with and without intervening soil building crops, each under three tillage methods (NT, PT and zone-till (ZT)) and three cover cropping systems (none, rye and vetch). Physical indicators measured were wet aggregate stability (WAS), available water capacity (AWC) and surface hardness (SH) and subsurface hardness (SSH). Soil biological indicators included organic matter (OM), active carbon (AC), potentially mineralizable nitrogen (PMN) and root disease potential (RDP). Chemical indicators included pH, P, K, Mg, Fe, Mn and Zn. Results from the Willsboro and Aurora sites showed significant tillage effects for several indicators including WAS, AWC, OM, AC, pH, P, K, Mg, Fe and Mn. Generally, the NT treatment had better indicator values than the PT treatments. At the Chazy site, WAS, AWC, OM, AC, pH, K and Mg showed significant differences for tillage and/or harvest method, also with NT showing better indicator values compared to PT and corn grain better than corn silage. Aggregate stability was on average 2.5 times higher in NT compared to PT treatments at Willsboro, Aurora and Chazy sites. OM was also 1.2, 1.1 and 1.5 times higher in NT compared to PT treatments at Willsboro, Aurora and Chazy sites, respectively. At the Geneva site WAS, SH, AC, PMN, pH, P, K and Zn showed significant tillage effects. The cover crop effect was only significant for SH and PMN measurements. Indicators that gave consistent performance across locations included WAS, OM and AC, while PMN and RDP were site and management dependent. The composite soil health index (CSHI) significantly differentiated between contrasting management practices. The CSHI for the Willsboro site was 71% for NT and 59% for PT, while at the Aurora site it was 61% for NT and 48% for PT after 15 years of tillage treatments.

Core Ideas The extraction protocol for “glomalin” extracts protein from a wide variety of sources. The term glomalin or glomalin‐related soil protein is inaccurate and limits the utility of the method. The extracted protein pool should be viewed more broadly as a soil health indicator of potentially available organic N. Increased interest in practical, routine evaluation of soil health has created a need for rapid and inexpensive indicators that reflect soil nitrogen (N) status. Here we propose a soil protein measurement as an indicator of a functionally relevant and sensitive pool of organic N that can be rapidly quantified in soil testing laboratories. The procedure is based on a method that was historically used to measure “glomalin,” a pool putatively of arbuscular mycorrhizal fungal origin. Laboratory validation experiments demonstrate that the procedure extracts proteins from a wide range of sources, not just glomalin, and that continued use of the term glomalin is inaccurate and limits the application of the method. Therefore, we propose that the pool of proteins extracted by this method can be viewed more broadly as a soil health indicator that reflects the primary pool of organically bound N in soil and thus as potentially available organic N. We provide a laboratory protocol that details autoclaving soil in a neutral sodium citrate buffer solution followed by clarification and protein quantification steps.

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