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Research on nitrogen (N) mineralization from organic residues is important to understand N cycling in soils. Here we review research on factors controlling net N mineralization as well as research on laboratory and field modeling efforts, with the objective of highlighting areas with opportunities for additional research. Among the factors controlling net N mineralization are organic composition of the residue, soil temperature and water content, drying and rewetting events, and soil characteristics. Because C to N ratio of the residue cannot explain all the variability observed in N mineralization among residues, considerable effort has been dedicated to the identification of specific compounds that play critical roles in N mineralization. Spectroscopic techniques are promising tools to further identify these compounds. Many studies have evaluated the effect of temperature and soil water content on N mineralization, but most have concentrated on mineralization from soil organic matter, not from organic residues. Additional work should be conducted with different organic residues, paying particular attention to the interaction between soil temperature and water content. One- and two-pool exponential models have been used to model N mineralization under laboratory conditions, but some drawbacks make it difficult to identify definite pools of mineralizable N. Fixing rate constants has been used as a way to eliminate some of these drawbacks when modeling N mineralization from soil organic matter, and may be useful for modeling N mineralization from organic residues. Additional work with more complex simulation models is needed to simulate both gross N mineralization and immobilization to better estimate net N mineralized from organic residues.

The amount of N mineralized or immobilized during the decomposition of a crop residue will influence the amount of N available for crop uptake and will ultimately impact N‐management practices and groundwater quality. The objective of this work was to determine quantitative relationship(s) between a crop residue's chemical properties and the potential net amount of N that would mineralize in a season. Eight experiments (six from the literature and two conducted by the authors) were combined to determine general relationships between net N mineralized and residue chemical characteristics. Regression analysis indicated that 75 and 72% of the variability in the measured amounts of N mineralized in the eight experiments could be explained using either the C/N ratio or the square‐root transformation of N concentration of the residue, respectively. The break point between net N mineralization and net immobilization was calculated to be at a C/N ratio of 40, which corresponds to a N concentration of about 10 g N/kg (assuming residue C is 400 g/kg). Eighty percent of the variability in the amount of N mineralized could be explained by a regression equation that included N and the lignin‐to‐N ratio as independent variables. The fitted equations provide estimates of the maximum amount of N that potentially will mineralize in a season from incorporated crop residues of different N contents.

Growing a legume cover crop in place of fallow in a winter wheat (Triticum aestivum L.)–fallow system can provide protection against erosion while adding N to the soil. However, water use by legumes may reduce subsequent wheat yield. This study was conducted to quantify the effect of varying legume termination dates on available soil water content at wheat planting and subsequent wheat yield in the central Great Plains. Four legumes [Austrian winter pea, Pisum sativum L. subsp. sativum var. arvense (L.) Poir.; spring field pea, P. sativum L.; black lentil, Lens culinaris Medikus; hairy vetch, Vicia villosa Roth.) were grown at Akron, CO, as spring crops from 1994 to 1999. Legumes were planted in early April and terminated at 2‐wk intervals (four termination dates), generally starting in early June. Wheat was planted in September in the terminated legume plots, and yields were compared with wheat yields from conventional till wheat–fallow. Generally there were no significant differences in available soil water at wheat planting due to legume type. Soil water at wheat planting was reduced by 55 mm when legumes were terminated early and by 104 mm when legumes were terminated late, compared with soil water in fallowed plots that were conventionally tilled. Average wheat yield was linearly correlated with average available soil water at wheat planting, with the relationship varying from year to year depending on evaporative demand and precipitation in April, May, and June. The cost in water use by legumes and subsequent decrease in wheat yield may be too great to justify use of legumes as fallow cover crops in wheat–fallow systems in semiarid environments.

Winter wheat (Triticum aestivum L.) is the most common dryland crop grown in the central Great Plains. Producers in this region include fallow in the rotation to minimize yield variability due to erratic precipitation. However, fallow degrades soil quality by increasing erosion potential and loss of organic matter. Fortunately, minimum‐till production systems and residue management improve water use efficiency by plants, thus producers can crop more frequently. We evaluated eight rotations comprised of various sequences of winter wheat (W), corn (Zea mays L.) (C), proso millet (Panicum miliaceum L.) (M), sunflower (Hettanthus annum L.) (S), and fallow (F) in comparison to W‐F at Akron CO. Our goal was to identify rotations that can replace W‐F to minimize the frequency of fallow. The soil was a Weld silt loam (Aridic Paleustoll). Continuously cropping with W‐C‐M and W‐M almost doubled total grain yield compared with the conventional system of W‐ F. Other rotations such as W‐C‐F, W‐C‐S‐F, and W‐C‐M‐F yielded >60% more on an annualized basis than W‐F. Winter wheat yield increased with longer time intervals between wheat crops. Sunflower yielded the most when grown only once every 4 yr; more frequent cropping favored diseases. Sunflower reduced yield of the following crop, especially during dry years. Yield variability was highest with corn and sunflower, whereas proso millet showed the least variability. Producers can manage yield variability by diversifying crops in the rotation, as annualized yield variability of W‐M and W‐C‐M was similar to W‐F. With residue maintenance and minimum tillage, producers can crop more frequently, thus increasing land productivity while minimizing the frequency of fallow in this semiarid region. Research Question Since the 1930s, winter wheat‐fallow has been the prevalent crop rotation for the semiarid Central Great Plains. Because available water is usually the most limiting resource, producers rely on fallow to minimize the impact of erratic precipitation on grain production. However, fallow degrades soil quality by increasing erosion and loss of organic matter. Development of minimum‐till production systems has altered the water relations in our agroecosystems. Minimizing tillage leaves more crop residue on the soil surface, subsequently increasing precipitation storage and water use efficiency of crops. Thus, with minimum‐till systems, more intensive cropping is possible in the central Great Plains. This study evaluated cropping systems composed of various sequences of winter wheat (W), corn (G), proso millet (M), sunflower (S), and fallow (F), including continuous cropping. Our goal was to identify rotations that may be successful alternatives to W‐F. Literature Summary With reduced‐till systems, several crops have been successful in a wheat‐summer crop‐fallow rotation in this region, including proso millet, corn, and grain sorghum. In addition, longer rotations with three crops in 4 yr, such as W‐C‐M‐ F, are also successful and have increased land productivity by 70%. Another p...

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Continuous cropping or decreasing the frequency of summer fallow (F) in cereal‐based dryland rotations may have benefits other than greater water utilization and erosion control. We hypothesized that rotations with no fallow or minimum fallow frequency can produce more biomass and cover than the traditional winter wheat (Triticum aestivum L.)‐summer fallow systems (W‐F), and ultimately, greater amounts of soil organic matter (SOM). To this end, we evaluated changes in various pools of SOM at the 0‐ to 5‐ and 0‐ to 15‐cm depths on a Weld loam (fine, smectitic, mesic aridic Paleustolls) that were caused by (i) decreasing fallow or increasing cropping intensities, (ii) specific rotations of the same length but with different crop sequencing, and (iii) accumulated residue and roots from reduced‐ or notillage from 1993 to 1997. Total soil organic carbon (SOC) and N for the 0‐ to 5‐cm depth increased by =20% with continuous cropping rotations compared with W‐F rotations. Particulate organic matter‐carbon (POM‐C) doubled, while POM‐N, and soluble organic C (OC) increased by one third for the same comparison. At the 0‐ to 15‐cm depth, SOC, POM‐C, and POM‐N did not differ among systems with fallow, nor among systems with cropping intensities greater than W‐F. Thus, significant differences always existed between W‐F and continuous cropping. Generally, fallow had a negative influence on SOC accumulation, and continuous cropping a positive influence on surface SOM. Changes in SOC did not correlate with yields in the five‐year comparison of this ongoing study.

N made available to crops that follow legumes in rotation. An estimate of soil mineralizable N is needed to determine crop While fertilization guides use total organic matter and needs for N fertilizer. The objective of this research was to estimate previous crop as indicators of N mineralization for the soil net N mineralization in soils maintained in continuous corn (Zea mays L.) (CC), corn-soybean [Glycine max (L.) Merr.] (CS), and coming season, a variety of direct and indirect lab methcorn-soybean-wheat (Triticum aestivum L.)/alfalfa (Medicago sativa ods may be used for more precise predictions (Fox and L.)-alfalfa (CSWA) rotations that have been managed since 1990 Piekielek, 1978; Hong et al., 1990). Laboratory tests with zero N (0N), low N (LN), and high N (HN) fertilization. Soil allow compositing and homogenizing soil samples to samples were taken from 0-to 20-cm depth in plots planted to corn decrease the standard deviation and required replicain 1998. In order to produce more realistic time-series data of net N tion. Aerobic incubation for 120 to 252 d is commonly mineralization, soils were incubated in filtration units in a variableused to estimate the size and decay rates of mineraliztemperature incubator (VTI) that mimicked field soil temperatures able N pools (Stanford and Smith, 1972; Cabrera and under a growing corn canopy. Rotation and N fertilization significantly Kissel, 1988). Temperature and matric potential of incuaffected net N mineralization in soil samples. Cumulative net N minerbated soils affect the rate and cumulative N mineralized. alized in a 189-d field temperature incubation averaged 133 Ϯ 6 kg ha Ϫ1 in CC, 142 Ϯ 5 kg ha Ϫ1 in CS, and 189 Ϯ 5 kg ha Ϫ1 in CSWA. Within ordinary field soil matric potentials from Ϫ1.85 Across rotations, average net N mineralized was 166 Ϯ 9 kg ha Ϫ1 in to Ϫ0.01 MPa, temperature has a greater influence on 0N plots, 147 Ϯ 10 kg ha Ϫ1 in LN plots, and 152 Ϯ 10 kg ha Ϫ1 in N mineralization than does matric potential (Zak et al., HN plots. Inclusion of a legume, particularly alfalfa, in the rotation 1999). Most N mineralization laboratory experiments increased net N mineralized. Generally, more net N was mineralized are incubated at 35ЊC, considered the ideal temperature from plots receiving no fertilizer N than from soil with a history of for maximum N mineralization. Nitrogen mineralized N fertilization. Variable-temperature incubation produced realistic in laboratory incubations at 35ЊC represents potential time-series data with low sample variability.