Manure management: implications for greenhouse gas emissions

The effects of compaction on soil porosity and soil water relations are likely to influence substrate availability and microbial activity under fluctuating soil moisture conditions We conducted a short laboratory incubation to investigate the effects of soil compaction on substrate availability and biogenic gas (CO 2 and N 2 O) production during the drying and rewetting of a fine-loamy soil Prior to initiating the drying and wetting treatments, CO 2 production (−10 kPa soil water content) from uncompacted soil was 23 times that of compacted soil and corresponded with higher concentrations of microbial biomass C (MBC) and dissolved organic C (DOC) In contrast, N 2 O production was 67 times higher in compacted than uncompacted soil at field capacity Soil aeration rather than substrate availability (eg NO 3 − and DOC) appeared to be the most important factor affecting N 2 O production during this phase The drying of compacted soil resulted in an initial increase in CO 2 production and a nearly two-fold higher average rate of C mineralization at maximum dryness (owing to a higher water-filled pore space [WFPS]) compared to uncompacted soil During the drying phase, N 2 O production was markedly reduced (by 93–96%) in both soils, though total N 2 O production remained slightly higher in compacted than uncompacted soil The increase in CO 2 production during the first 24 h following rewetting of dry soil was about 25 times higher in uncompacted soil and corresponded with a much greater release of DOC than in compacted soil MBC appeared to be the source of the DOC released from uncompacted soil but not from compacted soil The production of N 2 O during the first 24 h following rewetting of dry soil was nearly 20 times higher in compacted than uncompacted soil Our results suggest that N 2 O production from compacted soil was primarily the result of denitrification, which was limited by substrates (especially NO 3 − ) made available during drying and rewetting and occurred rapidly after the onset of anoxic conditions during the rewetting phase In contrast, N 2 O production from uncompacted soil appeared to be primarily the product of nitrification that was largely associated with an accumulation of NO 3 − following rewetting of dry soil Irrespective of compaction, the response to drying and rewetting was greater for N 2 O production than for CO 2 production

Compaction effects on CO2 and N2O production during drying and rewetting of soil

Management options to reduce nitrous oxide emissions from intensively grazed pastures: A review

Addressing the impact of soil heterogeneities on soil water patterns at the field scale involves measuring and/or modeling water content evolution with fine spatial and temporal resolution. The presence of stones introduces difficulties for both the measurement of the water content by TDR probes and the soil hydraulic properties. In this study, the role of stones was explicitly considered for interpreting TDR-based measurements of water content and its variability in the field, as well as for adjusting the hydraulic properties of the fine fraction to that of the bulk soil including stones. The in situ TDR measurements of the bulk dielectric constant were converted to the bulk water content by adopting an approach explicitly accounting for the contribution of the volumetric fraction of stones and their dielectric properties. The water content evolution was also simulated by using laboratory-based hydraulic properties as input in a numerical model. The soil hydraulic properties of the bulk soil (stones plus fine soil) were deduced from the soil core-based hydraulic properties according to the volumetric proportion of stones in the bulk soil. For the hydraulic conductivity, we assumed that the effect of stones is simply to reduce the cross-sectional area of the bulk sample available for flow, by assuming that, on the average, the areal fraction of stones is equal to the volume fraction of stones. In modeling soil water content, we also analyzed the effect of reducing hydraulic conductivity and water retention on the evaporation process and thus on the areal distribution of soil water content. Finally, the effect of stoniness was considered as a possible explanation of the differences frequently observed between the measured hydraulic behavior and that estimated by using PTFs. Overall, both the measured and PTF-based hydraulic properties, when scaled for stoniness, proved to be adequate for describing the average evolution of the water content. However, measured hydraulic properties describe the areal distribution of water contents in the field better than PTF.

Measuring and modeling water content in stony soils

A recent study (Wolf et al., 2010) suggests that short—lived pulses of N2O emission during spring thaw dominate the annual N2O budget and that grazing decreases N2O emissions during the spring thaw. To verify this we conducted year—round N2O flux measurements from June 2010 to May 2011 in Tianshan alpine grassland in central Asia. No pulse emissions of N2O were found at grazing management sites and nitrogen addition sites during the spring thaw. The contribution of the spring thaw to the total annual N2O budget was small and accounted for only 6.6% of the annual fluxes, with winter emissions accounting for 16.7% and growing season emissions accounting for 76.7%. The difference in N2O emissions attributable to grazing management was not significant (P > 0.05). Nitrogen input tended to increase N2O emissions at N addition sites during the grass growing season compared with those at unfertilized sites. N2O fluxes showed a significant correlation with air temperature and also with both soil temperature and soil water content at 10 cm depth.

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No significant nitrous oxide emissions during spring thaw under grazing and nitrogen addition in an alpine grassland

Understanding the physical and hydraulic properties of the vadose zone is important for modeling land use effects on groundwater quality. This study used a variety of characterization methods to derive conceptual understanding and estimates of hydraulic properties of a coarse alluvial gravel vadose zone in New Zealand. Sandy gravel (SG) material constituted approximately 90% of the vadose zone, with the remainder comprising sand lenses and open-framework gravels. The gravel content of the SG material was approximately 70% (v/v) (range 68–73%). The water content of the bulk SG material (43 samples) ranged from 3.5 to 13.9%. The average bulk density of the SG material was 2.20 g/cm 3 (range 2.00–2.33 g/cm 3 ) giving an average calculated porosity of 17%. The average porosity of the open-framework gravels was 34% and these gravels were often coated with 2- to 3-mm-thick deposits of amorphous Fe and Mn oxides. Neutron probe (NP) depth profiles indicated unsteady conditions, with variable water contents with depth and time reflecting the vertical heterogeneity and the variably saturated state of the vadose zone. Time series NP data to 3 m indicated water content in the alluvial gravels responded quickly to soil drainage events, and saturation variability was greater in the sand lenses and the SG material immediately underlying. When compared with derived water retention curves, variability in the water content equated to significant fluctuation in unsaturated hydraulic conductivity ( K unsat ). Tension infiltrometer measurements were variable but were within the range of the K unsat estimates from site-average particle size distribution data. The gravel-transformed, texture-based models used to estimate saturated water content values in this study appeared to underestimate the measured values.

Characterization and Estimation of Hydraulic Properties in an Alluvial Gravel Vadose Zone

Nitrous oxide (N2O) emissions to the atmosphere from grazed pasture can be high, especially from urine-affected areas When pastoral soils are damaged by animal treading, N2O emissions may increase In New Zealand, autumn-sown winter forage crops are often grown as a break-crop prior to re-sowing pasture When these crops are grazed in situ over winter (as is common in New Zealand) there is high risk of soil damage from animal treading as soil moisture contents are often high at this time of year Moreover, the risk of soil damage during grazing increases when intensive tillage practices are used to establish these forage crops Consequently, winter grazed forage crops may be an important source of N2O emissions from intensive pastoral farming systems, and these emissions may be affected by the type of tillage used to establish them We conducted a replicated field experiment to measure the effects of simulated cattle grazing (mowing followed by simulated treading and the application of synthetic urine) at three soil moisture contents ( field capacity) on measured N2O emissions from soil under an autumn (March) sown winter forage crop (triticale) established with three levels of tillage intensity: (a) intensive, IT, (b) minimum, MT, or (c) no tillage, NT In all treatments, bulk density in the top 75 cm of the soil was unaffected by treading when simulated grazing occurred at field capacity, and by 10% in the MT plots trodden at > field capacity Treading did not significantly increase the bulk density in the NT plots Emissions of N2O from the tillage treatments decreased in the order IT > MT > NT N2O emissions were greatest from plots that were trodden at > field capacity and least from plots trodden at field capacity The N2O emission from urine-amended NT plots that were trodden at < field capacity was 20 kg ha−1 over 90 days (025% of the total urine N applied) Decreasing the intensity of tillage used to establish crops and restricting grazing when soils are wet are two of the most effective ways to minimise the risk of high N2O emissions from grazed winter forage crops

Effects of tillage, simulated cattle grazing and soil moisture on N2O emissions from a winter forage crop

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H.E. Barlow

Papers

Characterization and Estimation of Hydraulic Properties in an Alluvial Gravel Vadose Zone

Effects of tillage, simulated cattle grazing and soil moisture on N2O emissions from a winter forage crop

Winter grazing of forages - soil moisture and tillage methods impact nitrous oxide emissions and dry matter production