Diurnal changes in nitric oxide emissions from conventional tillage and pasture sites in the Amazon Basin: influence of soil temperature

We have investigated a subset of restoration practices applied to a degraded pasture at Fazenda Nova Vida, a 22000 ha cattle ranch in Rond^onia, Brazil. Nitric oxide (NO) and carbon dioxide (CO₂) emissions from soils were measured in conventional tillage and current pasture sites to assess N and C losses. Mean daily NO emissions from tilled plots were at least twice those from the pasture. Nitric oxide emissions from the tilled sites showed a strong diurnal pattern, while those from the pasture sites did not. Mean daytime NO emissions from the tilled sites were 9.7 g NO-N m^–2 h^–1, while mean nighttime emissions were 29.7 g NO-N m^–2 h^–1. In the pasture sites, NO emissions were 7.6 g NO-N m^–2 h^–1 during the day, and 7.7 g NO-N m^–2 h^–1 at night. Surface soil temperature was a good inverse predictor (r ²=0.75) of NO emissions from the tilled sites. Carbon dioxide emissions from the tilled sites were generally larger than CO₂ emissions from the pasture sites. The mean CO₂ emission rate from the tilled sites was 179 mg C m^–2 h^–1, while it was 123 mg C m^–2 h^–1 from the pasture sites. There was no distinct diurnal pattern for CO₂ emissions. We found that the very high temperatures measured at the soil surface in the tillage plots, in the range of 40–45°C, reduced the rate of NO emission. The reduction in NO emissions may be because of the sensitivity of autotrophic nitrifiers to high temperatures. This study provides insights on how land-use change may alter regional NO fluxes by exposing certain microbial communities to extreme environmental conditions. Future studies of NO emissions in tropical agricultural systems where soils are bare for extend periods need to make diurnal measurements or the daily fluxes will be substantially underestimated.