Greenhouse Gases Emissions And Carbon Budget From Vegetable Ecosystems

Vegetable field is characterized by high harvest index, intensively plantated with high nitrogen fertilizer input, thus being an important source of greenhouse gases (GHGs) emissions. The study was performed in an intensive managed vegetable production region with four typical leafy vegetable rotations in southeast China from28December in2009to26December in2010. The main objectives were to simultaneously measure N2O, CH4and CO2fluxes and soil profile N2O, CH4and CO2concentrations, turnover rate and influence factors, to estimate the net ecosystem carbon budget (NECB) and greenhouse gases intensity (GHGI) on crop seasonal scale. In addition, the effect of biochar on greenhouse gas mitigations was also probed into through a pot experiment. The four consecutive rotations were established as follows:Celery-Tung choy-Baby bok choy-Amaranth (C-T-Bb-A), Choy sum-Celery-Tung choy-Bok choy (Cs-C-T-Bc), Garland chrysanthemum-Tung choy-Bok choy (G-T-Bc), Celery-Choy sum-Lettuce-Bok choy (C-Cs-L-Bc). The results were presented as follows:1. Vegetable field ecosystem is an important source of greenhouse gases emissions. N2O emissions from intensively managed vegetable fields were characterized by high fluxes that varied with cropping systems. Annual cumulative N2O emissions were237.7kg N ha-1from the G-T-Bc,137.2kg N ha-1from the C-T-Bb-A,100.9kg N ha-1from the Cs-C-T-Bc,56.4kg N ha-1from the C-Cs-L-Bc and29.2kg N ha-1from the bare fallow, respectively. Except for the C-Cs-L-Bc rotation, the cumulative N2O emissions from the rotation fields were significantly higher than that from the bare fallow. Vegetable field had no significant effect on CH4emissions. Annual cumulative CH4emissions were9.0kg C ha-1,13.9kg C ha-1,18.8kg C ha-1,12.1kg C ha-1and16.1kg C ha-1from the bare fallow, C-T-Bb-A, Cs-C-T-Bc, G-TBc and C-Cs-L-Bc, respectively. The annual N2O emission factor ranged from1.2%to5.0%, averaging with2.6%. N2O emissions were significantly linear correlated with total N input rates, and total N fertilizer application rate explained35.5%of annual N2O emissions. Soil temperature and soil moisture were crucial environment factors affecting N2O emissions, and soil NO3--N and NH4+-N had no significant correlations with N2O fluxes. Frequent year-round monitoring of N2O fluxes is essential for better constraint of the national N2O budget.2. Soil profile N2O, CH4and CO2concentration, diffusion fluxes and turnover rate showed greater spatial variability among profile layers in vegetable ecosystem. N2O concentration increased with soil depth in0-30cm profile, but its concentration showed increase or decrease in30-50cm profile and presented disaccord compared with15-30cm. N2O of30-50cm profile primly output, and0-7cm primly input. CH4concentrations increased with soil depth in0-50cm profile. CH4of30-50cm profile primly output, and0-15cm primly input. Lower CO2concentration in below soil profile and higher in upper soil profile. CO2of15-50cm profile primly output, and0-7cm primly input. N application and tillage managements all affected N2O profile distributions. N application significantly increased N2O concentrations in each soil profile, promoted N2O diffusion fluxes and accelerated N2O turnover rate, but those had no obvious effect on CO2soil profile distributions. In contrast with N fertilizer applications, the effect of tillage on CO2concentration, diffusion fluxes and turnover rate were greater. The effect of N application and tillage managements on CH4soil distributions and turnover rate were not obvious compared with that on N2O and CO2.3. NECB and δSOC from the four vegetable rotation fields all showed carbon sequestration at crop seasonal time scale. The δSOC from four vegetable rotations ranged from0.01t C ha-1to0.40t C ha-1. Application manure into vegetable field was important for increasing soil carbon stock. GWP, net GWP, GHGI and corresponding indexes of mGWP, net mGWP and mGHGI considering carbon emissions induced from field management and chemical material input all showed nearly consistent changes among the rotations and among the vegetables within each rotation. The global warming potential ranged from36～131Mg CO2eq. ha-1for mGWP,26～109Mg CO2eq. ha-1for net GWP and35～129Mg CO2eq. ha-1for net mGWP. Greenhouse gases intensity ranged from0.17～0.42kg CO2equiv. kg-1veg. for GHGI and0.22～0.49kg CO2equiv. kg-1veg. for mGHGI. The mGWP, net GWP, net mGWP, GHGI and mGHGI were dominated by the GWP resulting from N2O emissions. Increasing fertilizer use efficiency and adoption of best practices are effective measures for decreasing global warming potential in vegetable field management.4. A outside pot experiment with planting choy sum and amaranth was performed to estimate the effect of maize straw biochar application on greenhouse gas mitigations. Eight treatments included control (CK) with planting vegetable without fertilizer application and seven equal N rate treatments (400kg N ha-1) which were Urea(400kg N ha-1), UM1(combination urea with manure1), UM2(combination urea with manure2), UB1(combination urea with biochar1), UB2(combination urea with biochar2), UM1B (combination urea with manure1and biochar addition) and UM2B (combination urea with manure2and biochar addition). The results showed that UB1, UB2, UM1B and UM2B significantly decreased N2O emission by77%to86%, while the UM1and UM2did not show significant N2O emission difference in comparison with Urea. Biochar amendment or manure application had no significant effect on CH4emissions. On average, UM1B and UM2B significantly enhanced vegetable production by32%,48%and28%as compared to Urea, average UM1/UM2and average UB1/UB2, respectively. N2O-N emission factors with biochar application were0.4%-0.7%, while those without biochar being2.5%～3.2%. The most effective combination was biochar at30Mg ha-1. Biochar application can significantly reduce N2O emissions while maintaining vegetable production.Therfore, it is desired for further study on GHGs emissions and carbon budget from vegetable field ecosystems. It is also crucial for further studies on net GWP and GHGI from vegetable ecosystems.