Dynamics of nitrogen and carbon cycling associated with greenhouse gas emissions in the salt-affected soils.

Abstract

Salinity is one of the most severe environmental factors limiting the productivity of aquaculture and agriculture. The worldwide area of salt-affected soils is predicted to become even more widespread in the future due to climate change and sea-level rise. However, the soil nitrogen and carbon dynamics associated with soil-induced gas emissions under salinity are not well understood. The main objective of this study was to investigate changes of soil carbon and nitrogen cycling associated with greenhouse gas emissions, plant growth and fertilizer recovery under effects of different salinity levels. This study addressed research issues with the following main objectives. The main aim of the study reported in Chapter 2 was to analyse greenhouse gas production from different soils with different times of lid closure and to assess the effects of different activation time on gas emissions from soils. The results showed that the 20-min sampling interval at the closure time of maximum 80 minutes had good results with less variance either for soil types or monitored gases. Lengthening activation times for the incubation study may affect emission rates due to differences in soil properties. The study in Chapter 3 examined the effects of salinity and additional sources of nitrogen and carbon on soil nitrogen and carbon cycling in an acid sulphate soil (ASS) and an alluvial soil. The findings of this study demonstrated that salinity significantly decreased N2O emissions from the acid sulphate soil but did not affect emissions from the alluvial soil. The addition of glucose and nitrate enhanced N2O production in both salt-affected soils. This investigation indicated that salinity altered the carbon and nitrogen cycles in the acid sulphate soil; it recommends that future fertiliser and crop management will need to account for the changed nutrient cycling caused by saline water intrusion and climate change. The objective of the study reported in Chapter 4 was to identify a relationship between induced-soil gas emissions and the abundance of denitrification genes in a salt-affected soil. Increased salinity caused a decrease in both flux and cumulation of the N2O-N production and soil respiration from the incubated soil. The study result also showed that elevated salinity increased the denitrifying genes in the incubated acid sulphate soil. Abundance of the nir genes was usually high between the first and second week of incubation, while number of copies of the nosZ gene were significantly low at those times. Another study presented in Chapter 5 investigated changes in soil properties, the dynamics of N and its effects on rice growth and yield under different salinity levels by using a 15N label fertilizer technique. Flooding soils for two weeks by saline water greatly decreased rice yield and yield components in the acid sulphate soil. High salinity significantly lowered the recovery of fertilizer N by rice plants, especially in the acid sulphate soil where the crop did not produce any grain. The loss of fertilizer nitrogen was highly controlled by the interaction effect of soil types and salinity. Findings from the thesis substantially and originally contribute to the literature on salt-affected soils and will assist in developing new managemental interventions and strategies for soils where increased salinity is a real possibility in the future