Organic manure is an ideal alternative fertilizer to provide phosphorus (P) but is not fully recycled in subtropical China. In order to identify if it can replace chemical P fertilizer, a 35-year field trail in a paddy soil under double-rice cropping system was conducted to assess the effects of substituting chemical P fertilizer with pig manure (NKM) on rice yield, phosphorus use efficiency (PUE) and P balance. The N, P and K input under NKM was 1.2, 0.8 and 1.2 times of the combined chemical fertilizer treatment (NPK), respectively. The NKM treatment reached the same level of grain yield with NPK after 20 years’ application, and showed significantly 4.0% decreased double-rice grain yield compared with NPK over the 35 years. The NKM treatment reduced the crop P uptake leading to decreased PUE compared with NPK. Long-term P budget showed that NKM may result in higher potential of P loss than NPK. Thus, substituting chemical P fertilizer with organic manure under this rate of nutrient input slightly sacrificed the crop yield and may increase the P loss. Considering the benefits of soil fertility, adjusting the substitution rate with a more balanced NPK input might be alternative in subtropical China.

Methane (CH₄) is one of the key greenhouse gases (GHGs) in the atmosphere with current concentration of 1859 ppb in 2017 due to climate change and anthropogenic activities. Rivers are of increasing concern due to sources of atmospheric CH₄. However, knowledge and data limitations exist for field studies of subtropical agricultural river catchments, particularly in southern China. The headspace balance method and the diffusion model method were employed to assess spatiotemporal variations of CH₄ diffusive fluxes from April 2015 to January 2016 in four order reaches (S1, S2, S3, and S4) of the Tuojia River, Hunan, China. Results indicated that both the dissolved concentrations and diffusive fluxes of CH₄ showed obvious spatiotemporal variations. The observed mean concentration and diffusive flux of CH₄ were 0.40 ± 0.02 μmol L⁻¹ and 41.19 ± 2.50 µg m⁻² h⁻¹, respectively, showing the river to be a strong source of atmospheric CH₄. The CH₄ diffusive fluxes during the rice-growing seasons were significantly greater than the winter fallow season (an increase of 80.26%). The spatial distribution of CH₄ diffusive fluxes increased gradually from (17.58 ± 1.42) to (55.56 ± 4.32) µg m⁻² h⁻¹ due to the organic and nutrient loading into the river waterbodies, with the maximum value at location S2 and the minimum value at location S1. Correlation analysis showed that the CH₄ diffusive fluxes exhibited a positive relationship with the dissolved organic carbon (DOC), salinity, and water temperature (WT), while a negative correlation occurred between CH₄ diffusive fluxes and the dissolved oxygen (DO) concentration, as well as the pH value. Our findings highlighted that a good understanding of exogenous nutrient loading in agricultural catchments will clarify the influence of human activities on river water quality and then constrain the global CH₄ budget.