Effects Of Earthworms On Soil Nitrogen And Phosphorus In Upland Rice-Wheat Rotation Agroecosystem

It is well known that earthworm can affect nutrient cycling in terrestrial ecosystems significantly through modifying soil physical, chemical and biological properties. However, much of the current knowledge on the role of earthworms in nutrient cycling and dynamics were concluded from either short duration or laboratory simulations. Few extended studies were carried out about the effects of earthworms on these processes under ecosystem-level and field conditions, particularly far less on the upland rice-wheat ecosystem with corn residues manipulations. Therefore, a long-term field experiment was used to determine ecosystem-level fluxes of N and P, and potential N leaching contributed by earthworms in a natural agro-ecosystem.Earthworms (Metaphire guillelmi) were inoculated in the plots with a rotation of upland rice - winter wheat and application of corn residues. The effects of earthworm were investigated throughout the 6-year experiment, which were the mineralization of soil N, the change of microbial biomass, the dynamics of organic N and inorganic P fractions. 15 experimental field plots (2.8m × 1m × 0.6m) were laid out in a completely randomized block design with five treatments: corn residues incorporated with soil, no earthworms (I); corn residues incorporated, earthworms added (IE); corn residues mulched on the soil, no earthworms (M); and corn residues mulched, earthworms added (ME), and no corn residues, no earthworms (CK). There were 3 replicates per treatment. The main results were as follows:1 Overall ANOVA, of which earthworm, residues and time were sources of variances, indicated that earthworm factor affected most soil functional indices ( P<0.05 ) . Among the three sources of variance of ANOVA (earthworm, residues and time), earthworm accounted for the most of the total variations in the content of soil microbial biomass carbon (MBC)(51%), acid-hydrolyzable ammonium N (AM) (67%) and sonicate/NaOH P ( SON/NaOH-P) (37%) . These may be due to the earthworm movement, includingswallow, digestion and egestion. Residues factor accounted for the most of total variation in total organic carbon (TOC), total N (TN), mineral N (NO₃^--N, NH₄⁺-N and NO₃^--N+NH₄^--N), microbial biomass N (MBN), total acid hydrolysable N (THN), Olsen-P and NaOH-Pi. Time factor accounted for total P (TP), MBC/MBN, amino sugar N (AS), amino acid N (AA) and acic-hydrolyzable unknown N (HUN), phosphatese, Resin-P, NaHCO3-P, HC1-P and Residual P. It indicates that both residues and time were important factors affecting most soil functions. Although residues or time usually interacted with earthworm significantly in the ANOVA of nearly all indices mentioned above (p<0.01), but this interactions didn't change the trends of influences of earthworms on the soil N and P, which reflected the important function of earthworm in soil ecology.2 Earthworms could enhance soil N mineralization greatly in lab experiment. Soil NO₃^--N, total inorganic N (NO₃^--N+NH₄⁺-N), cumulative net nitrification, cumulative net ammonification and cumulative net mineralization, net nitrification rate, and net mineralization rate were all increased by earthworm evidently ( P<0.05 )in no corn residues application treatment. When corn residues were manipulated (mulch or incorporation), soil cumulative net ammonification and NH₄⁺-N (14d or 70d) were both increased by earthworms. Whereas only few significance were found in the certain periods or certain treatments in field experiment, the general trends were same as the lab experiment. It was shown that earthworms could enhance the mineralization and the total N of plant. And it also contributed to improve soil available N with decomposing corn residues.3 Earthworms increased the leachate volume by more than 30%, although the increase was not significant (P>0.05). Earthworms also increased mineral N losses, and significantly increased by 15% (P<0.05) in ME treatment. One of the most interesting results was that Urea-¹5N losses were decreased by 16.4-16.6% by the presence of earthworms,...