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One goal of soil C sequestration is to increase the mass of C stored in agricultural soils. Reducing soil disturbance, e.g., no-till management, facilitates soil fungal growth and results in higher C sequestration rates; however, the specific mechanisms associated with short-term plant residue C and N retention are less clear. We applied [sup.13]C- and [sup.15]N-enriched grain sorghum [Sorghum bicolor (L.) Moench] residue to no-till (NT) and conventional tillage (CT) soils, and measured the [sup.13]C and [sup.15]N retention in the soil and in aggregate fractions, along with soil microbial dynamics, during a growing season. The added plant residue mineralized rapidly in both tillage systems, with similar decomposition kinetics, as indicated by [sup.13]C data. Mass balance calculations indicated that approximately 70% of the added [sup.13]C was mineralized to C[O.sub.2] by 40 d. Total Gram positive and Gram negative bacteria and fungal phospholipid fatty acids were higher under NT 0 to 5 cm during the most active period of residue mineralization compared with the CT 0- to 5- or 5- to 15-cm depths. No changes were observed in the NT 5- to 15-cm depth. The >1000-[micro]m aggregate size class retained the most [sup.13]C, regardless of tillage. The NT >1000-[micro]m aggregates retained more [sup.15]N at the end of the experiment than other NT and CT aggregates size classes. Data obtained indicate higher biological activity associated with NT soils than under CT, and increased retention of plant residue C and N in macroaggregates. Abbreviations: CT, conventional tillage; [Gm.sup.+], Gram positive; [Gm.sup.-], Gram negative; NT, no-till; PLFA, phospholipid fatty acids; R+, residue added; R-, no residue added; SOC, soil organic carbon; TC, total carbon; TN, total nitrogen; VPDB, Vienna PeeDee Belemnite.

Predicting in situ nitrogen (N) mineralization has been one of the greatest challenges to improving N management in agriculture. This study investigated the effect of tillage and residual N on soil N supplying capacity and evaluated the relationship between measured and estimated mineralizable N. The experiment was established in 1990 on a moderately well-drained Kennebec silt loam (Fine-silty, mixed, superactive mesic Cumulic Hapludoll) with continuous corn (Zea mays L.). The study was a split-split plot design replicated four times. The main plot treatment was tillage (no-tillage [NT] and conventional tillage [CT]), the subplot treatment was N source (manure and N[H.sub.4]N[O.sub.3] fertilizer [F]), and the sub-subplot treatment was the length of residual period. Residual N was studied 1 yr after cessation of a 10-yr N application ([R.sub.1]) and 6 yr after cessation of a 5-yr N application ([R.sub.6]). Measured in situ N mineralization ([N.sub.min]), laboratory potentially mineralizable N ([N.sub.o]), and estimated N mineralization under field conditions ([N.sub.estimated]) were evaluated. Nitrogen mineralization was studied in situ in an unplanted, sheltered area. Samples were collected from 0- to 5-, 5- to 15-, and 15- to 30-cm depths. No-tillage and manure significantly increased soil total N, [N.sub.min], and [N.sub.o]. The combination of NT and manure significantly increased [N.sub.o] in both [R.sub.1] and [R.sub.6]. High correlation was observed between [N.sub.min] and [N.sub.estimated] for 0 to 5 cm (r = 0.79) and for 0 to 30 cm (r = 0.77). No-tillage and manure sustained soil N 6 yr after discontinued N application. Potential mineralizable N, for site specific conditions could be used to estimate in situ N mineralization after adjustment to field conditions (soil water and temperature).

Understanding the sorption and desorption behavior of N[H.sup.+.sub.4] in soils associated with animal waste is important because of the potential for the formation of N[O.sup.-.sub.3] and subsequent leaching that affects ground water quality. Batch equilibration experiments were conducted to evaluate the sorption and desorption of N[H.sup.+.sub.4] in two soils exposed to a complex matrix (liquid swine waste) anti a simple matrix [aqueous solution of 0.01 M Ca[Cl.sub.2], containing [(N[H.sub.4]).sub.2] S[O.sub.4]]. Kennebec silt loam (fine-silty, mixed, mesic Cumulic Hapludolls) and Haynie very line sandy loam (coarse-silty, mixed, calcareous, mesic Mollie Udifluvents) were used. This study revealed that the sorption and desorption behavior of N[H.sup.+.sub.4] in soils exposed to [(N[H.sub.4]).sub.2]S[O.sub.4] solutions with a 0.01 M Ca[Cl.sub.2], matrix is significantly different from that in soils exposed to liquid swine waste. Faster sorption rate, lower sorption capacity, and higher desorption capability were observed for N[H.sup.+.sub.4] in soils exposed to the [(N[H.sub.4]).sub.2]S[O.sub.4] solution compared with soils exposed to the liquid swine waste. Sequential extraction could nut extract nonexchangeable N[H.sup.+.sub.4] in both soils exposed to liquid swine waste, while a significant amount of nonexchangeable N[H.sup.+.sup.4] was extracted from the two soils that were initially exposed to the [(N[H.sub.4]).sub.2]S[O.sub.4] solutions. The high dissolved organic C (DOC) content coupled with the high pH in swine waste appears to stimulate the sorption and retard desorption of N[H.sup.+.sub.4] in the two soils. This study revealed that batch equilibrium studies using solutions with simple matrixes may underestimate the sorption or overestimate desorption of N[H.sup.+.sub.4] in soils associated with swine waste.

The availability of N limits productivity in tallgrass prairie. Spring burning is common because it results in greater plant productivity despite reducing net N mineralization. To better explain how burning affects inorganic N availability in tallgrass prairie, the partitioning of [sup.15]N among plant and soil pools was measured in June and August 1994. Approximately 2.5 [micro]g N [g.sup.-1] soil was injected as either N[H.sup.4] or N[O.sup.3] to a depth of 15 cm within cores in burned and unburned prairie. Cores were removed from the field 6 d alter injection, and [sup.15]N recovery in plant and soil N pools was determined. No more than 14% of the applied [sup.15]N remained in inorganic form 6 d after application. The largest portion of the applied [sup.15]N (35-80%) was recovered in the soil organic nitrogen pool ([N.sub.0]). Burning significantly increased the immobilization of both N[H.sup.4] and N[O.sup.3] within [N.sup.0], and microbial biomass accounted for [greater than or equal to] 50% of the [sup.15]N recovered in [N.sub.0]. Accumulation of [sup.15]N in plants accounted for [less than or equal to] 35% of the applied [sup.15]N with a majority recovered from roots. Burning had little effect on [sup.15]N recovery in plants; however, [sup.15]N accumulations in roots were significantly greater when N[O.sub.3] was used. Results indicate that immobilization within soil organic matter (SOM) controls the availability of both N[H.sup.4] and N[O.sub.3] to plants. Increased immobilization in soils with burning probably results largely from increased microbial N demand resulting from greater litter inputs with wider C to N ratios. Further research is needed to determine if abiotic mechanisms for N immobilization also significantly influence N availability in prairie soils.

In agricultural systems, maintenance of soil organic matter (SOM) has long been recognized as a strategy to reduce soil degradation. No-tillage and manure amendments are management practices that can increase SOM content and improve soil aggregation. We investigated the effects of 10-yr of different tillage systems and N sources on soil aggregate-size distribution and aggregate-associated C and N. The study was a split-plot design replicated four rimes. The main plot treatment was tillage (no-tillage, NT; conventional tillage, CT) and the subplot treatment was N source (manure, M; N[H.sub.4]N[O.sub.3] ferrilizer, F). The experiment was established in 1990 on a moderately well-drained Kennebec silt loam (Fine-silty, mixed, superacrive mesic Cumulic Hapludoll) with continuous corn (Zea mays L.). In 1999, soil samples were collected (0- to 5-cm depth) from the field treatments and separated into four aggregate-size classes (>2000, 250-2000, 53-250, and 20-53 pm) by wet sieving. Labile C and N content of all aggregate-size fractions were measured using 28-d laboratory incubations of intact and crushed aggregates. No-tillage and M treatments significantly increased total C and N and the formation of macroaggregates. Conventional tillage in comparison with NT significantly reduced macroaggregates with a significant redistribution of aggregates into microaggregates. Aggregate protected labile C and N were significantly greater for macroaggregates, (>2000 and 250-2000 [micro]m) than microaggregates (53-250 and 20-53 [micro]m) and greater for M than F indicating physical protection of labile C within macroaggregates. No-tillage and M a Ione each significantly increased soil aggregation and aggregate-associated C and N; however, NT and M together further improved soil aggregation and aggregate-protected C and N.

The article examines the feasibility and the efficacy of carbon sequestration, or the storage of carbon in soil to reduce levels of atmospheric carbon dioxide. Discussion focuses on the use of agricultural systems which can help maximize the amounts of carbon that soils can store.

The denitrification potential of subsoils and aquifers must be characterized to assess the ultimate fate of soil N. This experiment was conducted to study changes in the subsurface distribution and activity of denitrifying bacteria as a result of cultivation. We examined soil profiles for their capacity (N addition) and potential (N + C addition) to denitrify as well as for denitrifying bacterial numbers in a Reading silt loam (fine, mixed, mesic Typic Argiudoll) soil under grassland and cultivation. Denitrifying enzyme activity was undetectable throughout the subsurface of soil profiles at both sites. Overall, the cultivated site had significantly higher N[[O.sub.3].sup.-] concentrations, denitrifier populations, and denitrification potential. Denitrifying bacteria were stratified in the vadose zone at the prairie site, with lowest numbers and lowest denitrification potential occurring at the interface of the water table. Although C addition enhanced denitrification at this site, the profile was not nearly as limited by C as that of the cultivated site. The results demonstrated that increased N inputs and cultivation during approximately 50 yr changed the denitrifying population and denitrification potential in the vadose and saturated zones of soils.

An evaluation of the effect of N fertilization mowing and burning on the microbial biomass (MBM) of tallgrass prairie proves that MBM is important for the conservation of N in tallgrass prairie. Mowing decreases the amount of microbial C (MC) and microbial N (MN) in vegetation, while the addition of nitrogen increases MN and reduces the amount of MC. However, the presence of MC and MN in tallgrass prairies emphasizes the need for the cycling of these nutrients.

Fertilizer N management can mitigate N2O emissions but complex soil‐weather conditions modulate the mitigation potential. Conditional inference tree (CIT) is a machine learning method able to untangle complex interactions while providing an interpretable model. The goals of this study were (a) to assess the effect of N fertilizer on N2O emissions, and to use CIT to identify (b) the main soil‐weather drivers of daily N2O hot moments and (c) fertilizer management options to mitigate them. The study was conducted in 2 yr in no‐till corn (Zea mays L.) with seven combinations of N source and placement tested. Daily N2O emissions were measured with vented chambers, and soil temperature and water‐filled pore space (WFPS) were measured near the chambers on the same days of gas sampling. Overall, 2013 was drier with lower N2O emissions than 2014. Cumulative N2O losses differed across treatments and years, with broadcast emitting more in 2014 than in 2013, and only subsurface‐banded fertilizer with a nitrification inhibitor (NI) consistently abated N2O losses. The main hot moment conditions were within ∼80 d of fertilizer application when soil temperature >15 °C and WFPS >57%. Under these conditions, NI abated losses by 50% compared with fertilizer alone. The machine learning approach used here could be used in larger datasets to elucidate environment‐specific drivers of N2O hot moments and potential fertilizer mitigation practices under different soil, weather, and management conditions. Core Ideas: Nitrous oxide losses are affected by complex soil‐weather and management conditions.Broadcast urea, coated urea, and urea‐ammonium nitrate emitted more N2O in a wetter vs. drier year.Nitrification inhibitor was the only N management to consistently abate N2O losses.Nitrification inhibitor decreased N2O losses by ∼58% overall and by 50% during hot moments.Hot moments were identified as within ∼80 DAA, when soil temperature >15 °C and WFPS >57%. [ABSTRACT FROM AUTHOR]