Effects of solid oxygen fertilizers and biochars on nitrous oxide production from agricultural soils in Florida.

Elevated levels of nitrous oxide (N2O) emissions are a matter of concern in agricultural soils especially when flooding (hypoxic conditions) results from over irrigation or frequent rains. This study is the first to report the use of two solid oxygen fertilizers (SOFs, calcium peroxide and magnesium peroxide) to reduce N2O production in mineral and organic soils amended with N fertilizer in a short-term laboratory incubation besides two biochars. In general, organic soil had greater N2O production than mineral soil. Soils amended with nitrogen fertilizer exhibited increased N2O production, by 74 times in mineral soil and 2 times in organic soil. Both solid oxygen fertilizers in mineral soil (98-99%) and calcium peroxide in organic soil (25%) successfully reduced N2O production than corresponding N fertilized treatments. Additionally, a greater level of available nitrate-N (52-57 and 225 mg kg-1 in mineral and organic soil, respectively) was recorded with the solid oxygen fertilizers. Corn residue biochar with N fertilizer increased N2O production in mineral soil but decreased in organic soil, while pine bark biochar with N did not affect the N2O production in either soil. Depending on soil, appropriate SOFs applied were able to reduce N2O production and maintain greater nitrate-N levels in flooded soil. Thus, solid oxygen fertilizers can potentially be used as an effective way to reduce N2O emission from hypoxic soil in agricultural production systems.

Duration and frequency of drainage and flooding events interactively affect soil biogeochemistry and N flux in subtropical peat soils.

With the demand for restoration and future prediction of climate change effects, subtropical peatlands are expected to be subjected to hydrologic regimes with variable duration and frequency of drained and flooded conditions, but knowledge of their interactive effects on soil biogeochemistry and emission of greenhouse gases including nitrous oxide (N2O) is largely limited. The objective of this study was to investigate how the duration and frequency of drainage and flooding events interactively influence soil biogeochemical properties and denitrification and related net N2O production rates following rewetting. Surface soils are susceptible to different hydrologic regimes. Significantly higher pH, extractable organic carbon (ext. OC), ammonium (NH4+-N), denitrification enzyme activity (DEA), but lower nitrate (NO3--N), microbial biomass C and N were observed when the peat soils were under flooded conditions compared to drained conditions. Two-week and four-week drainage or flooding duration did not result in statistically significant differences in soil biogeochemical properties. A 24-week prolonged drainage led to an accumulation of NO3--N and a significantly lower pH. Soil microbial biomass and fungal:bacterial abundance likely increased with the frequency of drainage-flooding cycles. Significant differences in denitrification and net N2O production rates following reflooding were mainly found in the surface soils. Structural equation modeling indicated that hydroperiod and water-filled pore space (WFPS) prior to reflooding is likely to control denitrification and net N2O production through its regulation of NO3--N and activity of microorganisms involved in denitrification while higher drainage-flooding frequency decreases the availability of organic C and NO3--N for denitrification. Our results also suggest high NO3--N and low pH within peat soils caused by prolonged drainage likely leads to a significant N2O emission pulse following reflooding. For peat soils subjected to frequent drainage-flooding cycles, N2O emission pulses following reflooding would decrease with time, attributing to the loss of substrates for denitrification.

Rate of warming affects temperature sensitivity of anaerobic peat decomposition and greenhouse gas production.

Temperature sensitivity of anaerobic carbon mineralization in wetlands remains poorly represented in most climate models and is especially unconstrained for warmer subtropical and tropical systems which account for a large proportion of global methane emissions. Several studies of experimental warming have documented thermal acclimation of soil respiration involving adjustments in microbial physiology or carbon use efficiency (CUE), with an initial decline in CUE with warming followed by a partial recovery in CUE at a later stage. The variable CUE implies that the rate of warming may impact microbial acclimation and the rate of carbon-dioxide (CO2 ) and methane (CH4 ) production. Here, we assessed the effects of warming rate on the decomposition of subtropical peats, by applying either a large single-step (10°C within a day) or a slow ramping (0.1°C/day for 100 days) temperature increase. The extent of thermal acclimation was tested by monitoring CO2 and CH4 production, CUE, and microbial biomass. Total gaseous C loss, CUE, and MBC were greater in the slow (ramp) warming treatment. However, greater values of CH4 -C:CO2 -C ratios lead to a greater global warming potential in the fast (step) warming treatment. The effect of gradual warming on decomposition was more pronounced in recalcitrant and nutrient-limited soils. Stable carbon isotopes of CH4 and CO2 further indicated the possibility of different carbon processing pathways under the contrasting warming rates. Different responses in fast vs. slow warming treatment combined with different endpoints may indicate alternate pathways with long-term consequences. Incorporations of experimental results into organic matter decomposition models suggest that parameter uncertainties in CUE and CH4 -C:CO2 -C ratios have a larger impact on long-term soil organic carbon and global warming potential than uncertainty in model structure, and shows that particular rates of warming are central to understand the response of wetland soils to global climate change.

Seasonal patterns of nitrogen cycling in subtropical short-hydroperiod wetlands: Effects of precipitation and restoration.

In the event of increased frequency of extreme wet or dry events resulting from climate change, it becomes more important to understand the temporal dynamics of soil nitrogen (N) processes in ecosystems. Here, seasonal patterns of N cycling were characterized in subtropical wetlands in Everglades National Park, Florida, USA. Two restored sites and one reference site with different nutrient status, soil depth, and vegetation communities, were selected. Soil available N, microbial biomass, potential N mineralization and denitrification rates, enzyme activities of leucine aminopeptidase (LAP) and N-acetyl-ß-d-glucosaminidase (NAG) were measured across the wet and dry seasons from 2010 to 2011. In general, most N processes were significantly correlated with soil water contents (P<0.05) which reflected the precipitation regime. The lower elevation and shallower soil (2-3cm depth) at the restored site may contribute to their higher soil water contents compared to the reference site with ~10cm soil depth, which further led to the earlier peaks of microbial biomass at the two restored sites. Potential N mineralization was positively correlated with LAP at the restored sites whereas with NAG at the reference site (P<0.05), implying that different vegetation composition may provide varying substrates for soil microbes. The build-up of nitrate in the dry spring of 2011 induced a pulse of denitrification after rewetting by a sudden rainfall, implying the presence of a hot moment of denitrification during the dry-rewetting transition period. The decrease of MBC:MBN ratio from dry to wet season indicates a possible microbial composition shift from fungi to bacteria, shedding lights on the potential contribution of fugal groups to denitrification in the dry season. Our study highlight that even under the same climate regime, the small-scale variations could affect the seasonal patterns of N cycling.

Nitrous oxide production and consumption by denitrification in a grassland: Effects of grazing and hydrology.

Denitrification is generally recognized as a major mechanism contributing to nitrous oxide (N2O) production, and is the only known biological process for N2O consumption. Understanding factors controlling N2O production and consumption during denitrification will provide insights into N2O emission variability, and potentially predict capacity of soils to serve as sinks or sources of N2O. This study investigated the effects of hydrology and grazing on N2O production and consumption in a grassland based agricultural watershed. A batch incubation study was conducted on soils (0-10 cm) collected along a hydrological gradient representing isolated wetland (Center), transient zone (Edge) and pasture upland (Upland), from both grazed and ungrazed areas. Production and consumption potentials of N2O were quantified on soils under four treatments, including (i) ambient condition, and amended with (ii) NO3(-), (iii) glucose-C, and (iv) NO3(-) +glucose-C. The impacts of grazing on N2O production and consumption were not observed. Soils in hydrologically distinct zones responded differently to N2O production and consumption. Under ambient conditions, both production and consumption rates of Edge soils were higher than those observed for Center and Upland soils. Results of amended incubations suggested NO3(-) was a key factor limiting N2O production and consumption rates in all hydrological zones. Over 5-d incubation with NO3(-) amendment, cumulative production and consumption of N2O for Center soils were 1.6 and 3.3 times higher than Edge soils, and 3.6 and 7.6 times higher than Upland soils, respectively. However, cumulative N2O net production for Edge soils was the highest, with 2 to 3 times higher than Upland and Center soils. Our results suggest that the transient areas between wetland and upland are likely to be "hot spots" of N2O emissions in this ecosystem. Wetlands within agricultural landscapes can potentially function to reduce both NO3(-) leaching and N2O emissions.