Impact of carbon and nitrogen application in paddy-soil ecosystem: 13,14C labeling, zymography, pH mapping and PLFA

Rice (Oryza sativa L.), a major cereal crop, is cultivated on more than 140 million hectares worldwide. Consequently, rice growing paddy fields are major consumers of nitrogen (N) fertilizer and the biggest users of agricultural water. Compared to other non-flooded agricultural lands, paddy soils contain 12%–58% higher soil organic matter (SOM) content, which makes them an important carbon (C) sink. Rice straw retention, nitrogen fertilizer application and over-flooded water are the three most important factors responsible for the higher C sink in paddy fields. Together, they have great impact on rice-soil ecosystems. This thesis therefore presents three studies within the confines of these key factors. The two studies (Chapter 2 and 3) were designed to reveal: 1) the effects of N fertilization, and 2) the long term effects of rice straw retention on the distribution of photosynthates in varies soil C pools. Soil C and N availed by N fertilizer application and rice straw retention affect microbial composition and activities, resulting in altered SOM decomposition and plant assimilates allocation. Experiments conducted focused on the interactions between C and N availabilities and the consequent effects on rhizodeposition and microbial community in paddy soil. Using carboxymethyl cellulose (CMC) as long term rice straw decomposition mimic, treatments: CMC (+C), (NH4)2SO4 (+N), their combination (+CN), and unfertilized soil (control) were designed. Rice were continuously labeled with 13CO2 and the tracer (13C)incorporated into both above- and belowground plant biomass, SOM, dissolved organic matter (DOC), microbial biomass (MBC), and phospholipid fatty acids (PLFAs) was quantified. The long term degradation of rice straw as mimicked by single CMC application (+C) led to mobilization of a 3% of total N from SOM and a positive N priming effect. This finding supported the microbial N mining hypothesis. The highest rice yield increase occurred in +CN treatment despite smallest root biomass and lowest assimilation of 13C into roots, DOC, SOM, and MBC. Additionally, +CN altered microbial community composition. Specifically, +CN application decreased 1) Gram-positive (G+)/ gram-negative (G-) ratios, and 2) G+ bacteria and fungi abundance. Contrary, G- and actinomycetes were stimulated by N fertilization. Fertilization and plant growth stage are the two factors that explained 81% of the variance in the microbial communities. Fertilization was responsible for 36.5% of the variance in the composition of microorganisms. Flooding, as another key factor in paddy field, creates anaerobic conditions, which changed root morphology and soil physiochemical properties such as root iron plaque, rhzodepositions and pH. Since the impact of flooding on paddy fields remain unknown, we introduced triple combination of 14C imaging, pH mapping and zymography for the first time in paddy soils (Chapter 4). This combination enabled the water effects from root iron plaque, rhzodepositions and pH on five enzyme activities involved in carbon (C) (β-glucosidase, cellobiohydrolase，xylanase), nitrogen (N) (leucine aminopeptidase), and phosphorus (P) (phosphatase) cycling to be evaluated. Varying the H2O content from <25% to oversaturation, we confirmed the hypotheses: 1) flooding increases root biomass but decreases water use efficiency; 2) flooding increases rhzodeposition (14C) but decreases pH in both rhizosphere and bulk soil; 3) flooding is the dominant factor determining the spatial distribution patterns of enzyme activities. Through diffusion effects, flooding evenly distributed enzyme substrates. Through 3D mesh and contour map, we simultaneously evaluated the correlations of enzymes involved in C, N and P cycling successfully. The cancelling effect of flooding resulted in loss of several optimal combination peaks of C, N and P related enzymes through diffusion. This flooding effect ultimately narrowed the optimal combination area. Concluding, water effects improved formation of root iron plaque, increased rhzodepositions and decreased pH. This PhD thesis therefore introduced new concepts such as cancelling effects and developed new triple combination of enzyme zymography, 14C imaging and pH mapping approach. The study improved the understanding on how the three key factors: 1) rice straw retention, 2) N fertilization, and 3) flooding impact the rice-soil ecosystem, and enabled further guidance on countering the challenges brought about global climate change.