Combining automated and manual chambers to provide reliable estimates of N2O emissions in annual and perennial cropping systems

Abstract

Perennials can produce more biomass and partially replace annual crops. However, environmental benefits of perennials over annuals in terms of nitrous oxide (N₂O) emissions have rarely been compared in a long-term field experiment. By combining automatic and manual chamber methods, we aimed to develop reliable N₂O estimates from annual and perennial systems. We measured N₂O emissions from: i. perennial grass during renovation including spring barley as catch crop (SB/RG); ii. perennial grass-clover mixture (GC); iii. triticale monoculture (Trit). Results showed that cumulative N₂O emissions from SB/RG were higher than GC or Trit. The highest emission rate was measured for SB/RG (258.9 µg N₂ON m^-2 h^-1) after fertilization in spring. Increased N₂O emissions were also seen for a short period after direct grass seeding in August. For Trit, N₂O emissions increased after fertilization in March and ploughing in late September. In GC (fertilized with P and K), there was no N₂O peak after grass cutting. Both from manual and automatic chamber systems, “hot moments” of N₂O emissions contributed ∼16–79 % of cumulative emissions. By predicting hot moments and scheduling frequent measurements, manual chambers captured most of the N₂O dynamics. The results indicated that the hot moments of N₂O emissions were better quantified by automatic chambers, while some of the hot moments, for instance, fertilization and ploughing in Trit were accurately captured with manual chambers. Soil nitrate and ammonium were positively associated with N₂O emissions, whereas biomass N uptake was negatively associated. We conclude that perennial (GC) is a promising system for high biomass production with low environmental impact. Strategies such as growing spring barley as a catch crop, grass seeding with shallow tillage, and fertilization of newly seeded grass matching crop N demand are needed to reduce the higher risk for N losses.