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.

Phosphorus (P) is one of the main limiting nutrients in subtropical forest soils. Both soil type and aggregate structure affect the P sorption capacity of soil; thus, determining soil P supply and leaching characteristics. However, the mechanism of their interactions on soil P sorption and leaching at an aggregate level remains unclear. We classified soil aggregates from red soils and limestone soils in a subtropical forest via wet-sieving and carried out P isothermal sorption experiments. The P sorption maximum (Q_m), P sorption strength (K_L, K_F), P sorption index (PSI) and maximum buffer capacity (MBC) were obtained by fitting to Langmuir and Freundlich isotherm equations. Moreover, different P fractions were determined to estimate the degree of P sorption saturation (DPS) of aggregates. The results showed that the Q_m of the two soils were similar, but the sorption strength (K_L, K_F) and MBC of the limestone soil were higher than those of the red soil. Higher contents of free iron (Fe) oxide and amorphous aluminum (Al) oxide in the limestone soil may enhance the P sorption capacity and, thus, reduce P availability, resulting in a higher total P retention capacity than in the red soil. A higher content of complex Fe in red soil may reduce P sorption and, therefore, play a role in maintaining the supply capacity of soil-available P. The 0.25–0.5 mm aggregates of the two soils had the largest MBC among all aggregate sizes, and their P sorption and buffering capacity were stronger than other aggregates. The DPS of different aggregate sizes were all low, indicating that the soils of subtropical forests were in a state of P deficiency; thus, the risk of P leaching was low. The <0.1 mm aggregate in red soil had relatively high DPS and significantly lower PSI than the other aggregate sizes, indicating that it was more prone to P leaching. The results provide further insight into forest management to improve P availability and reduce P leaching in subtropical forest soils.

Commonly, well‐drained forest soils are net methane (CH₄) sinks, whereas poorly drained soils in groundwater discharge zones could contribute significant CH₄emissions. Climate change is projected to bring more summer precipitation extremes to subtropics, which may affect forest CH₄balance. Chinese forests often have a hilly topography with well‐drained hillslopes and pronounced groundwater discharge zones. Although different landscape elements are likely to have different source and sink functions for CH₄, the climatic influence on CH₄budget at the catchment scale remains poorly studied. Here, we measured CH₄fluxes over three years along a topographic gradient in a subtropical forested catchment in SW China. CH₄fluxes exhibited a clear spatial pattern indicating moderate CH₄uptake or emission from the well‐drained soils on the hillslope (−5.8 to +1.0 kg CH₄‐C·ha⁻¹·year⁻¹) and significant CH₄emission from the wetter soils in the groundwater discharge zone (94.3 to 479.5 kg CH₄‐C·ha⁻¹·year⁻¹). Despite its small area contribution to the catchment (0.6%), the groundwater discharge zone emitted substantial amounts of CH₄in monsoonal summers, affecting the whole‐catchment budget. Hillslope soils showed weaker CH₄uptake or even turned into a small CH₄source under wet climatic conditions. Estimates of catchment‐scale CH₄budgets indicate that subtropical forest catchments can tip from a net CH₄sink in drier years to a net CH₄source in wetter years, highlighting the importance of interannual climate variabilities. Our findings suggest that subtropical forests under projected climate change could contribute a net CH₄source with consequences for the regional CH₄balance.

Canonical ammonia-oxidizing archaea (AOA), ammonia-oxidizing bacteria (AOB) and complete-nitrifying bacteria (comammox) exist in a variety of ecosystems. However, little is known about AOA, AOB and comammox or their contributions to nitrification in the soils of heavily degraded and acidic mine regions. In the present study, the activity, richness, diversity and distribution patterns of AOA, AOB and comammox in the Siding mine area were investigated. Nemerow's multifactor pollution index (PN) values indicated that the soil in all three areas in the Siding mine area was highly contaminated by Cd, Pb, Zn, Mn and Cu. The AOA, AOB and comammox amoA gene copy numbers exhibited significant positive correlations with Pb and Zn levels and PN values, which indicated that the populations of AOA, AOB and comammox underwent adaptation and reproduction in response to pollution from multiple metals in the Siding mine area. Among them, the abundance of AOA was the highest, and AOA may survive better than AOB and comammox under such severely pollution-stressed and ammonia-limited conditions. The phyla Thaumarchaeota and Crenarchaeota may play vital roles in the soil ammonia oxidation process. Unlike AOA, AOB may use soil available phosphorus to help them compete for NH3 and other limiting nutrients with AOA and heterotrophs. Moreover, soil organic matter was the main factor influencing the species diversity of AOB, the β-diversity of AOB and comammox, and the community composition of AOA, AOB and comammox. Our research will help to explain the role and importance of AOA, AOB and comammox in the different ecological restoration regions in the Siding mine area.