| In the study of farmland soil organic matter dynamics,the quantification of rhizodeposited carbon(C)is essential.Although rhizodeposited C is very important for soil C and mineral nutrient cycling,rhizosphere deposition of carbon is difficult to quantify by conventional methods.Hence,rhizodeposited C remains the most indeterminate part of the terrestrial ecosystem soil carbon cycle.Nitrogen(N)fertilizer is the key factor to winter wheat production,but little is known about the effects of winter wheat rhizodeposited C on soil C pool.To solve this problem,we used pulsed 13CO2 labeling to investigate the effect of N fertilization on winter wheat photosynthesized C distribution and and input into belowground.Four growth stages of pot-grown winter wheat(tillering,elongation,anthesis and grain filling)were chosen for pulse-labeling with exposure to 13CO2,at two rates of nitrogen application(CK for no N application and +N for 250 kg N ha-1).The objectives of this study were to(?)quantify the photosynthesized C from plants to belowground and separate soil respiration at different growth stag;(?)investigate the amount of photosynthesized C input into belowground and the rhizosphere priming effects over whole growth season,and(?)fate of fertilizer N at different wheat growth stages.The main research results are as follows.(1)13C labeling of winter wheat at the grain filling stage with N fertilization may produce more uniformly 13C-labeled materials,while the materials were more highly 13C-enriched at the elongation stage with no N fertilization although the 13C values of the wheat materials were more variable.Compared with 13C labeling,differences in 15N excess among aboveground wheat components were much smaller under 15N labeling conditions.(2)The rhizosphere priming effect accelerated the original soil organic matter decomposition rate compared with unplanted soil,and the rhizosphere priming effect increased as the growth stages advanced(from 34.6%-44.4%at the elongation stage to 59.7%-76.9%at the grain filling stage).From the elongation to harvest,the rhizosphere priming effects of winter wheat led to an increase of decomposition of original soil C for +N treatment(58.6%;priming soil C amount 0.26 g C kg-1),which was significantly higher by 25%than CK treatment(46.9%;priming soil C amount 0.17 g C kg-1).(3)About 60%and 40%of the total N uptake by winter wheat was from fertilizer and soil,respectively.Basal and top-dressing fertilization contributed equally to the wheat N uptake.The recovery for fertilizer 15N via winter wheat increased with the crop development:from the anthesis to harvest the increase of recovery rate was 50%,and at the same period,the residual fertilizer 15N declined by 50%.After the winter wheat harvest,about 28.6%fertilizer 15N was left in the soil and 33.9%was lost in various pathways.(4)Allocation of 13C in the rhizosphere respiration was complete by day 26 after labelling.The majority of net photosynthesizd 13C was recovered in the aboveground,and proportion of net photosynthesizd 13C recovered(shoots + roots + soils + rhizosphere respiration)in the aboveground wheat increased from 58%-64%at the tillering stage to 86%-91%at the grain filling stage.In the roots,the proportion decreased from 21%-28%to 2-3%and in soil,from 1%-2%to 6%-7%,while in the rhizosphere respiration,the percentage peaked(17%-18%)at the elongation stage and then decreased to 6%-8%at the grain-filling stage.N fertilization decreased the proportion of C translocated into belowground by 20%,but increased the total fixed photosynthate C,and therefore increased the belowground C input(1.53 for CK vs.1.82 t C ha-1 for +N).Compared with soil receiving no N fertilizer,N fertilization decreased the proportion of assimilated 13C translocation into soil pool by 18%,but increased about 30 kg C ha-1 more than no N fertilized soil.The loss rate of basal fertilizer 15N was 21%higher than that of top-dressing fertilizer 15N,and this suggested that an increase of top-dressing N fertilization may decrease the N losses. |
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