No abstract

Max 300 words) 1 Land-use change is a prominent feature of the Anthropocene. Transitions between 2 natural and human-managed ecosystems affect biogeochemical cycles in many ways, but soil 3 processes are amongst the least understood. We used a global meta-analysis (62 studies, 4 1670 paired comparisons) to examine effects of land conversion on soil-atmosphere fluxes of 5 methane (CH 4 ) and nitrous oxide (N 2 O) from upland soils, and explored what soil and 6 environmental factors influenced these effects. Conversion from a natural ecosystem to any 7 anthropogenic land use increased soil CH 4 and N 2 O fluxes by 234 kg CO 2 -equivalents ha -1 y -8 1 , on average. Reverting to natural ecosystems did not fully reverse those effects, even after 9 80 years (except for CH 4 fluxes by -12 µg m -2 h -1 ). In general, neither the type of natural 10 ecosystem that was converted, nor the type of anthropogenic land use it was converted to, 11 affected the magnitude of increase in soil emissions. The exception to this is when natural 12 ecosystems were converted to pastures or croplands (emissions increased by +23 and +5 µg 13 CH 4 m -2 h -1 ). A complex suite of variables interacted to influencing CH 4 and N 2 O fluxes, but 14 availability of soil inorganic nitrogen (i.e. extractable ammonium and nitrate), texture, pH, 15 and microclimate were the strongest mediators of effects of land-use change. Land-use 16 changes in wetter ecosystems resulted in greater CH 4 fluxes, and effects of land-use change 17 on soil nitrate, total organic C, and pH emerged as the greatest drivers of changes in CH 4 18 fluxes. Effects of land-use change on N 2 O fluxes decreased in wetter ecosystems, and the 19 land-use change effect was regulated primarily via changes in soil inorganic N and water 20 content. Understanding the complicated effects of land-use changes on soil-atmosphere CH 4 21 and N 2 O fluxes, and the mechanisms underpinning such emissions, could inform land 22 management actions to mitigate increased greenhouse gas emissions after changing land uses.23

The soil organic carbon (SOC) and its fractions are good indicators of soil quality and environmental stability. Among the factors affecting SOC pool and fluxes in a watershed, land use changes and soil erosion are factors of importance. The differences in SOC and its fractions among different land uses can help understand the process of soil carbon dynamics. A study was conducted in Typic Ustochrepts of Northwest India to understand the impact of forest, grassland, agricultural and eroded lands on aggregate stability and SOC fractions. The undisturbed soil aggregates were sampled from different land uses in a watershed in Shiwaliks of lower Himalayas. The aggregate stability was determined by shaking the presaturated aggregates under water and by single simulated raindrop technique. The SOC, labile carbon, hotwater soluble carbon, particulate organic carbon and aggregate associated organic carbon were determined in aggregates of different sizes as well as in the bulk soils. The water stability of aggregates expressed as mean weight diameter (MWD) and stability index (SI SRT ) was highest in surface soils (0-15 cm) of grasslands followed by forest, agricultural and eroded lands. The WSA >2 mm (water stable aggregates >2 mm) were highest (17.3%) in grasslands and lowest (0.85%) in eroded lands. The eroded soils had 2.2, 7.4 and 3.4 times higher amount of micro-aggregates (WSA < 0.25 mm) than agricultural, forest and grassland soils, respectively. The SOC in surface soils significantly decreased by 27% in forest and 45% in agricultural land from that in grassland soils. In subsoil (15-30 cm), the SOC in eroded, agricultural and grasslands was statistically similar. The SOC stock in the subsoil (15-100 cm) was of significance. The grassland soils could store 41 Mgha -1 SOC stock compared to 31 Mg ha -1 in the subsurface layer. This difference widened in forestland, where subsoil contained 73.4% of total SOC stock in 100 cm soil profile. Among all the SOC fractions studied, labile carbon was mostly affected by erosion and was 91.6% lower in eroded than in grassland soils. The magnitude of aggregate associated organic carbon decreased with aggregate size in all the land uses. Among the SOC fractions, the aggregate stability under simulated raindrop impact could better be explained (R 2 =0.78) by hot water soluble carbon whereas the water stability of aggregate could better be explained (R 2 =0.69) by particulate organic carbon.

The lower Himalayan regions of north-west India experienced a severe land-use change in the recent past. A study was thus conducted to assess the effect of grassland, forest, agricultural and eroded land uses on soil aggregation, bulk density, pore size distribution and water retention and transmission characteristics. The soil samples were analysed for aggregate stability by shaking under water and water drop stability by using single simulated raindrop technique. The water-stable aggregates (WSA) >2 mm were highest (17Á3 per cent) in the surface layers of grassland, whereas the micro-aggregates (WSA < 0Á25 mm) were highest in eroded soils. The water drop stability followed the similar trend. It decreased with the increase in aggregate size. Being lowest in eroded soils, the soil organic carbon also showed an adverse effect of past land-use change. The bulk density was highest in eroded lands, being significantly higher for the individual aggregates than that of the bulk soils. The macroporosity (>150 mm) of eroded soils was significantly (p < 0Á05) lower than that of grassland and forest soils. The grassland soils retained the highest amount of water. Significant (p < 0Á05) effects of land use, soil depth and their interaction were observed in water retention at different soil water suctions. Eroded soils had significantly (p < 0Á05) lower water retention than grassland and forest soils. The saturated hydraulic conductivity and maximum water-holding capacity of eroded soils were sufficiently lower than those of forest and grassland soils. These indicated a degradation of soil physical attributes due to the conversion of natural ecosystems to farming system and increased erosion hazards in the lower Himalayan region of north-west India.

Future liquid fuel demand from renewable sources may, in part, be met by converting the seasonally wet portions of the landscape currently managed for soil and water conservation to perennial energy crops. However, this shift may increase nitrous oxide (N 2 O) emissions, thus limiting the carbon (C) benefits of energy crops. Particularly high emissions may occur during the transition period when the soil is disturbed, plants are establishing, and nitrate and water accumulation may favor emissions. We measured N 2 O emissions and associated environmental drivers during the transition of perennial grassland in a Conservation Reserve Program (CRP) to switchgrass (Panicum virgatum L.) and Miscanthus x giganteus in the bottom 3-ha of a watershed in the Ridge and Valley ecoregion of the northeastern United States. Replicated treatments of CRP (unconverted), unfertilized switchgrass (switchgrass), nitrogen (N) fertilized switchgrass (switchgrass-N), and Miscanthus were randomized in four blocks. Each plot was divided into shoulder, backslope, and footslope positions based on the slope and moisture gradient. Soil N 2 O flux, soil moisture, and soil mineral nitrogen availability were monitored during the growing season of 2013, the year after the land conversion. Growing season N 2 O flux showed a significant vegetation-bylandscape position interaction (P < 0.009). Switchgrass-N and Miscanthus treatments had 3 and 6-times higher cumulative flux respectively than the CRP in the footslope, but at other landscape positions fluxes were similar among land uses. A peak N 2 O emission event, contributing 26% of the cumulative flux, occurred after a 10.8-cm of rain during early June. Prolonged subsoil saturation coinciding with high mineral N concentration fueled N 2 O emission hot spots in the footslopes under energy crops. Our results suggest that mitigating N 2 O emissions during the transition of CRP to energy crops would mostly require a site-specific management of the footslopes.