Initial effects of fire and mechanical thinning on soil enzyme activity and nitrogen transformations in eight North American forest ecosystems

This study assessed the first-year effect of three ecosystem restoration treatments (prescribed fire, mechanical thinning, and their combination) on soil enzyme activity, soil N transformations, and C:N ratios of soil organic matter and mineral soil in eight North American forested ecosystems. The ecosystems we studied were part of the larger Fire and Fire Surrogate (FFS) network, and all had a history of frequent fire that has been altered by almost a century of organized fire suppression. Across all eight sites there were no statistically significant effects of the three manipulative treatments on phosphatase activity or chitinase activity; in contrast, at the network-scale phenol oxidase activity was reduced by fire alone, relative to the control. There was no significant network-scale effect of the three treatments on net N mineralization or net nitrification. Soil C:N ratio increased modestly after mechanical thinning, but not after prescribed fire or the combination of fire and thinning. There was a statistically significant reduction in forest floor C:N ratio as a result of all three treatments. Ordination of the differences between the treated and control areas indicated that fire alone resulted in greater changes in phenol oxidase activity and net nitrification than did the other two treatments. Large-scale restoration treatments such as those utilized in this study produce modest proximate effects on soil microbial activity and N transformations.     

Microbial biomass and bacterial functional diversity in forest soils: effects of organic matter removal, compaction, and vegetation control

The effects of organic matter removal, soil compaction, and vegetation control on soil microbial biomass carbon, nitrogen, C-to-N ratio, and functional diversity were examined in a 6-year loblolly pine plantation on a Coastal Plain site in eastern North Carolina, USA. This experimental plantation was established as part of the US Forest Service's Long Term Soil Productivity Study. Sampling was undertaken on eight treatments within each of three blocks. Treatments sampled included main 2×2 factorial treatments of organic matter removal (stem-only or complete tree plus forest floor) and compaction (none or severe) with split-plot treatment of vegetation control (none or total vegetation control). Two blocks were located on a somewhat poorly drained, fine-loamy, siliceous, thermic aeric Paleaquult (Lynchburg soil) and one on a moderately well drained, fine-loamy, siliceous, thermic aquic Paleudult (Goldsboro soil). Soil microbial C and N were positively related with soil C and N, respectively. Microbial C and N on the Lynchburg soil were higher than those on the Goldsboro soil. Organic matter removal decreased microbial N. Compaction reduced microbial C-to-N ratio. Vegetation control decreased microbial C and C-to-N ratio. The number of C compounds utilized by bacteria was not affected by soil type or treatment. However, soil types and treatments changed bacterial selections for a few C compounds on BIOLOG plates. Soil microbial properties varied more due to the natural soil differences (soil type) as compared with treatment-induced differences.     

Subsoil compaction effects on crops in Punjab, Pakistan:II. Root growth and nutrient uptake of wheat and sorghum

Crop yields can be reduced by soil compaction due to increased resistance to root growth, and decrease in water and nutrient use efficiencies. A field experiment was conducted during 1997–1998 and 1998–1999 on a sandy clay loam (fine-loamy, mixed, hyperthermic Typic Haplargids, USDA; Luvic Yermosol, FAO) to study subsoil compaction effects on root growth, nutrient uptake and chemical composition of wheat (Triticum aestivum L.) and sorghum (Sorghum bicolor L. Moench). Soil compaction was artificially created once at the start of the study. The 0.00–0.15 m soil was manually removed with a spade. The exposed layer was compacted with a mechanical compactor from 1.65 Mg m⁻³ (control plot) to a bulk density of 1.93 Mg m⁻³ (compacted plot). The topsoil was then again replaced above the compacted subsoil and levelled. Both compacted and control plots were hoed manually and levelled. Root length density, measured at flowering stage, decreased markedly with compaction during 1997–1998 but there was little effect during 1998–1999. The reduction in nutrient uptake by wheat due to compaction of the subsoil was 12–35% for N, 17–27% for P and up to 24% for K. The reduction in nutrient uptake in sorghum due to subsoil compaction was 23% for N, 16% for P, and 12% for K. Subsoil compaction increased N content in wheat grains in 1997–1998, but there was no effect on P and K contents of grains and N and P content of wheat straw or sorghum stover. During 1997–1998, K content of wheat straw was statistically higher in control treatment compared with compacted treatment. In 1998, P-content of sorghum leaves was higher in compacted treatment than uncompacted control. Root length density of wheat below 0.15 m depth was significantly reduced and was significantly and negatively correlated with soil bulk density. Therefore, appropriate measures such as periodic chiselling, controlled traffic, conservation tillage, and incorporating of crops with deep tap root system in rotation cycle is necessary to minimize the risks of subsoil compaction.     

Seasonal variation and fire effects on CH4, N2O and CO2 exchange in savanna soils of northern Australia

Tropical savanna ecosystems are a major contributor to global CO₂, CH₄ and N₂O greenhouse gas exchange. Savanna fire events represent large, discrete C emissions but the importance of ongoing soil-atmosphere gas exchange is less well understood. Seasonal rainfall and fire events are likely to impact upon savanna soil microbial processes involved in N₂O and CH₄ exchange. We measured soil CO₂, CH₄ and N₂O fluxes in savanna woodland (Eucalyptus tetrodonta/Eucalyptus miniata trees above sorghum grass) at Howard Springs, Australia over a 16 month period from October 2007 to January 2009 using manual chambers and a field-based gas chromatograph connected to automated chambers. The effect of fire on soil gas exchange was investigated through two controlled burns and protected unburnt areas. Fire is a frequent natural and management action in these savanna (every 1-2 years). There was no seasonal change and no fire effect upon soil N₂O exchange. Soil N₂O fluxes were very low, generally between −1.0 and 1.0 μg N m⁻² h⁻¹, and often below the minimum detection limit. There was an increase in soil NH₄⁺ in the months after the 2008 fire event, but no change in soil NO₃⁻. There was considerable nitrification in the early wet season but minimal nitrification at all other times.Savanna soil was generally a net CH₄ sink that equated to between −2.0 and −1.6 kg CH₄ ha⁻¹ y⁻¹ with no clear seasonal pattern in response to changing soil moisture conditions. Irrigation in the dry season significantly reduced soil gas diffusion and as a consequence soil CH₄ uptake. There were short periods of soil CH₄ emission, up to 20 μg C m⁻² h⁻¹, likely to have been caused by termite activity in, or beneath, automated chambers. Soil CO₂ fluxes showed a strong bimodal seasonal pattern, increasing fivefold from the dry into the wet season. Soil moisture showed a weak relationship with soil CH₄ fluxes, but a much stronger relationship with soil CO₂ fluxes, explaining up to 70% of the variation in unburnt treatments. Australian savanna soils are a small N₂O source, and possibly even a sink. Annual soil CH₄ flux measurements suggest that the 1.9 million km² of Australian savanna soils may provide a C sink of between −7.7 and −9.4 Tg CO₂-e per year. This sink estimate would offset potentially 10% of Australian transport related CO₂-e emissions. This CH₄ sink estimate does not include concurrent CH₄ emissions from termite mounds or ephemeral wetlands in Australian savannas.