The use of biochar as an agricultural soil improvement was tested in acid sulfate and sandy soils from Malaysia, cropped with rice and corn. Malaysia has an abundance of waste rice husks that could be used to produce biochar. Rice husk biochar was produced in a gasifier at a local mill in Kelantan as well as in the laboratory using a controlled, specially designed, top lift up draft system (Belonio unit). Rice husk biochar was applied once to both soils at two doses (2% and 5%), in a pot set up that was carried out for two cropping seasons. Positive and significant crop yield effects were observed for both soils, biochars and crops. The yield effects varied with biochar type and dosage, with soil type and over the cropping seasons. The yield increases observed for the sandy soil were tentatively attributed to significant increases in plant-available water contents (from 4%–5% to 7%–8%). The yield effects in the acid sulfate soil were likely a consequence of a combination of (i) alleviation of plant root stress by aluminum (Ca/Al molar ratios significantly increased, from around 1 to 3–5) and (ii) increases in CEC. The agricultural benefits of rice husk biochar application to Malaysian soils holds promise for its future use.

Overgrazing has caused serious grassland soil deterioration and significant soil organic carbon (SOC) losses. Controlled grazing is recommended for restoring grassland and enhancing SOC stocks. Studies have suggested that, in particular, labile organic carbon (C) is a sensitive indicator for soil degradation and SOC recovery, but few studies of the effect of different grassland management practices have been published so far. Here, we investigate the effects of long‐term grazing exclusion and short‐term rotational grazing with different grazing intensities on labile SOC, including hot water extractable organic C (HWEC) and particulate organic C (POC), as well as potential N mineralization (N _min ) rate in soils (0–10 cm) of two grasslands in Xilinhot, Inner Mongolia. Treatments include long‐term (34 years) exclusion, medium‐term (21 years) exclusion and continuous grazing (ambient grazing) at one of the sites, and we investigate no grazing, light grazing, moderate grazing and high‐grazing intensity at the study site for rotational grazing. Our results show that grazing exclusion significantly enhances labile SOC stocks and potential N _min rate compared to ambient grazing (e.g., POC stock at long‐ and medium‐term exclusion and ambient grazing are 5.71 ± 0.94 Mg C ha ⁻¹ , 3.25 ± 0.30 Mg C ha ⁻¹ and 2.12 ± 0.38 Mg C ha ⁻¹ , respectively). However, after two decades of exclusion, the potential for further C sequestration is limited. Five years of rotational grazing significantly enhances HWEC, but not POC and potential N _min rate, when comparing the highest grazing intensity (1.88 sheep ha ⁻¹ yr ⁻¹ ) with grazing exclusion during the same period. We conclude that rotational grazing may be a viable alternative to exclusion, offering a more rational use of grassland without causing soil C loss. We also show that HWEC is a particularly sensitive labile SOC indicator and is tightly correlated to POC and potential N _min rate. Highlights: We investigated effects of grazing exclusion and rotational grazing on labile soil organic carbon. Grazing exclusion increased labile SOC and the highest level appeared in medium‐term exclusion. Five years of high‐intensity rotational grazing increased HWEC with no significant effects on POC. Rotational grazing is a viable alternative to grazing exclusion management.

Chronically elevated nitrogen (N) deposition has led to severe nutrient imbalance in forest soils. Particularly in tropical and subtropical forest ecosystems, increasing N loading has aggravated phosphorus (P) limitation of biomass production, and has resulted in elevated emissions of nitrous oxide (N₂O) and reduced uptake of methane (CH₄), both of which are important greenhouse gases. Yet, the interactions of N and P and their effects on greenhouse gas emissions remain elusive. Here, we report N₂O and CH₄ emissions together with soil N and P data for a period of 18 months following a single P addition (79 kg P ha⁻¹, as NaH₂PO₄ powder) to an N-saturated, Masson pine-dominated forest soil at TieShanPing (TSP), Chongqing, south-western (SW) China. We observed a significant decline in both nitrate (NO₃⁻) concentrations in soil water (5 and 20 cm depths) and in soil N₂O emissions, following P application. We hypothesise that enhanced N uptake by plants in response to P addition, resulted in less available NO₃⁻ for denitrification. By contrast to most other forest ecosystems, TSP is a net source of CH₄. P addition significantly decreased CH₄ emissions and turned the soil from a net source into a net sink. Based on our observation and previous studies in South America and China, we believe that P addition relieves N inhibition of CH₄ oxidation. Within the 1.5 years after P addition, no significant increase of forest growth was observed and P stimulation of forest N uptake by understorey vegetation remains to be confirmed. Our study indicates that P fertilisation of N-saturated, subtropical forest soils may mitigate N₂O and CH₄ emissions, in addition to alleviating nutrient imbalances and reducing losses of N through NO₃⁻ leaching.

Forest soils in the warm-humid tropics significantly contribute to the regional greenhouse gas (GHG) budgets. However, spatial heterogeneity of GHG fluxes is often overlooked. Here, we present a study of N₂O and CH₄ fluxes over 1.5 years, along a topographic gradient in a rainforest catchment in Xishuangbanna, SW China. From the upper hillslope to the foot of the hillslope, and further to the flat groundwater discharge zone, we observed a decrease of N₂O emission associated with an increase of soil water-filled-pore-space (WFPS), which we tentatively attribute to more complete denitrification to N₂ at larger WFPS. In the well-drained soils on the hillslope, denitrification at anaerobic microsites or under transient water-saturation was the potential N₂O source. Negative CH₄ fluxes across the catchment indicated a net soil CH₄ sink. As the oxidation of atmospheric CH₄ is diffusion-limited, soil CH₄ consumption rates were negatively related to WFPS, reflecting the topographic control. Our observations also suggest that during dry seasons N₂O emission was significantly dampened (<10 μg N₂O-N m^-2 h^-1) and CH₄ uptake was strongly enhanced (83 μg CH₄-C m^-2 h^-1) relative to wet seasons (17 μg N₂O-N m^-2 h^-1 and 56 μg CH₄-C m^-2 h^-1). In a post-drought period, several rain episodes induced exceptionally high N₂O emissions (450 μg N₂O-N m^-2 h^-1) in the groundwater discharge zone, likely driven by flushing of labile organic carbon accumulated during drought. Considering the global warming potential associated with both GHGs, we found that N₂O emissions largely offset the C sink contributed by CH₄ uptake in soils (more significant in the groundwater discharge zone). Our study illustrates important topographic controls on N₂O and CH₄ fluxes in forest soils. With projected climate change in the tropics, weather extremes may interact with these controls in regulating forest GHG fluxes, which should be accounted for in future studies.