Net methane flux exchange in subtropical vegetable fields

Agricultural soil forms an important source-sink of atmospheric methane (CH₄). Studies on net CH₄ flux exchange in agricultural soil have mainly been related to rice, wheat, maize and other crops. However, field studies on vegetable fields have been seldom reported. This study was a year-round situ measurement of CH₄ flux in a typical subtropical vegetable field cultivated with cabbages using the static chamber/gas chromatography technique. The aims were to determine the characteristics of annual net exchange flux of CH₄ and the influencing factors, to estimate annual cumulative CH₄ flux, and to quantify errors in measured CH₄ flux. The CH₄ flux measurements were conducted on ridges and on inter-ridges in vegetable fields for period from January 1^st to December 8^th 2016. Environmental conditions were simultaneously observed during the measurement period. The results showed that vegetable fields constituted a weak sink of atmospheric CH₄, with annual mean flux of (-9.9±7.0) μg(C)·m^-2·h^-1 and annual cumulative flux of -0.84 kg(C)·hm^-2. High soil water content and nitrogen fertilizer application rate were probably the main reasons for the weak uptake of CH₄. The overall error in the annual cumulative CH₄ flux was -48%——16%. The main source of systematic error was in the flux calculation method used, which underestimated the flux by 32% on average. Random error was mainly produced by spatial variations of CH₄ flux, which was estimated at 16% at annual time scale. As a result, it was recommended that high numbers of spatial replications were used in conducting greenhouse gas flux measurements in agricultural soil in order to reduce random error. Another finding of the study was that CH₄ uptake measured on the ridge was significantly (P < 0.01) larger than that measured on the inter-ridge. This implied that it was better to simultaneously place chambers on areas with different field management practices; e.g. ridge and inter-ridge, row and inter-row, which efficiently avoided systematic error in measured flux.