| We have conducted these three experiments from2008to2011in semi-arid area of China.Experiment one:Plastic film mulching (PFM) has been extensively used for crop production since the1990s in the semi-arid Loess Plateau, Northwest China. The availability of soil inorganic P under PFM is poorly understood and hard to be predicted in this area. This paper determined the effects of PFM on P availability, soil P budget and topsoil P balance, and inorganic P fraction transformation on a calcareous soil with plastic-film-mulched spring wheat. This study was conducted in Zhonglianchuan, Yuzhong County, Gansu Province over two growing seasons in2009and2010. There were two treatments of spring wheat either mulched with plastic film (M) or non-mulched/bare (B). Individual plots,10m long x6m wide, were replicated three times in a randomized complete block design.Compared to the B treatment, grain yield in the M treatment increased by94.5%and73.4%in2009and2010, respectively; and straw biomass increased by62.6%and53.6%. P mass in aboveground spring wheat in the M treatment was significantly higher (P<0.05) than in the B treatment in both years. Moreover, P mass was significantly higher (P<0.05) in2010than the corresponding treatments in2009. As a result, P remaining in the M treatment was significantly lower (P<0.05) than in the B treatment in both years. In the2009growing season with less rainfall, the balance of available P was positive, but negative in2010with more rainfall. This can be explained on the basis that high crop biomass requires more soil available P in wet years than in dry years, which may decrease actual soil available P content in the wheat-growing period. In this trial, inorganic P fraction contents increased in both treatments after two consecutive growing seasons, but decreased for O-P (occluded Fe/Al bound P). In the M treatment, a significant positive correlation existed between Olsen-P and Ca2-P (dicalcium bound P), Al-P (aluminum bound P), Fe-P (iron bound P), Ca10-P (phosphorite bound P) in2009, and Ca2-P in2010, but no significant correlation occurred in the B treatment.Therefore, we suggest that PFM is beneficial for reducing P remaining in soil and promoting inorganic P fraction availability to crops in semiarid areas. Future research should focus on the mechanism for the effect of PFM with other crops on soil P availability and inorganic P fractions.Experiment Two:The poor and unsustainable for cropland soil are the mainly soil quality problem in arid and semi-arid area Loess Plateau of northwest China. The residue decomposition would be an important source of soil nutrient in semiarid area. This paper determined the effects of the water-harvesting on the residue decomposition and nutrient release, and the interaction between decomposition process and residue stoichiometric. This study was conducted in Zhonglianchuan, Yuzhong County, Gansu Province from June2008to April2011. Ten kinds of crops residue were enclosed into lmm nylon mesh bags and were randomly buried horizontal in soil about20cm below the soil surface in two treatments, which were water-harvesting (plastic-mulched ridge with both ridge and furrow60cm wide)(H) and conventional flat ground (F). Individual plots,12m long and1.8m wide, were replicated three times in a randomized complete block design.Results showed that the soil moisture in0-20cm layer in H treatments was significantly higher than in F treatments, but the soil temperature in10cm layer had no significant difference between two treatments. Moreover, the residue decomposition rate, nutrient content (containing C, N and P) and the nutrient release rate had no significant differences between H and F treatments in our study. So we can suggest that soil physical condition in ridge and furrow of water-harvesting, such as soil water and temperature, is not the major factor affecting the crop residue decomposition process in semi-arid area of China. The greatest loss of dry mass occurred from0-120days for corn, field pea, potato, and alfalfa straw residues, meanwhile from0-90days for all crop root residues in whole decomposition process. But for wheat straw, the greatest loess of dry biomass occurred at second decomposition stage (from120to300decomposition days). In all study periods, the amount of nutrient released by all crop residues followed the sequence C>N>P in both H and F treatment. For all crop residues, there had a highest release rate for C and N nutrient in first decomposition stage than other stages in both H and F treatments but except WS and WR. In H and F treatments, residue P release rate differed between different residues and decomposition stages. Understanding the process of residue decomposition and nutrient release are useful to design different crop rotation system in semiarid Loess Plateau of China. Residue decomposition rate had a significantly positive correlation with residue N and P concentration in straw and root residues in both H and F treatments in120and90days, respectively. In both treatments, there had a significantly negative correlation between N release rate and C:N, C:P in straw and root residues at this stage. Residue P release rate had a significantly positive correlation with N and P concentration, but had a significantly negative correlation with C:N and C:P in straw residue in both H and F treatments in first decomposition stages. We consider that the best plant residues quality relating to residue decomposition and residue nutrient mineralisation was total plant N, P concentration, C:P and C:N ratios in semiarid area of China.Experiment Three:This study determined the effects of the crop cultivation systems on soil C, N, P ecological stoichiometry and crop yield in semi-arid area. This study was conducted in Zhonglianchuan, Yuzhong County, Gansu Province from April2008to October2010. The experiments were laid out in a split plot design with two cropping patterns (crop rotation and continuous cropping)(main plots), six cultivation methods (sub-plots) and three replications. There were three cultivation methods for continuous cropping with spring wheat (C1), maize (C2) and potato (C3) in4years from2007to2010, respectively; And for crop rotation with spring wheat-potato-field pea-spring wheat (R1), maize-potato-field pea-maize (R2) and potato-field pea-potato-field pea (R3) in four years from2007to2010, respectively. The dimensions of individual sub-plots were6m wide×10m long. Planting date, seeding date and harvesting date for all crops grown in all treatments followed local practices.Results showed that the contents of soil C, N, P, C:N reduced, but N:P and C:P increased in all six treatments in28exprimental months. There had a logarithmic relationship between RC:N and the decreaing rate (Kc) of soil C, N in all non-mulched pltanted crop sequence. Moreover, decreasing of soil C and N was acclerated by plastic film mulched. There had a positive correlation between RC:N and Kc for soil C:N in non-mulched sequence. RC:P had a significantly negative correlation with Kc for soil P, but had a significantly positive correlation with Kc for soil C:P in all non-mulched crop sequences. In2010growing season, the yields of spring wheat and corn in crop roration treatments were significantly higher than in continuous cropping treatments, respectively.In a word, it was difficult to maintain the balance and stability of agroecosystem relying on self-repair in semi-arid area of China. Mean crop root C:N (RC:N) was the main driving factor for soil C:N changing. In currently cultivation practice, the imbalance of soil N and P would be the severe quality issues in this area. We also found crop yield was significanlty affected by previous crop in rotation system. |
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