| Alfalfa(Medicago sativa L.)crop rotation is considered to be an important link to promote combination of agriculture,grassland and livestock,and advance sustainable development in the semiarid Loess Plateau of China.Because of alfalfa grassland can bring deep soil drying and the worry about it may cause low productivity of succession crops,the alfalfa crop rotation system with alfalfa as the principal part has not been widely recognized and implemented.Therefore,in order to alleviate the drying of deep soil by alfalfa Grassland,and promote succession crops to maintain a higher production level,we compared farmland after alfalfa grassland standing for 30 years with the conventional farmland nearby.Soil moisture recovery,yield levels,and soil carbon,nitrogen and phosphorus trends were analyzed.We imposed 4 treatments: alfalfa crops rotation farmland plus fertilization(MF),alfalfa crops rotation farmland without fertilization(MO),conventional farmland plus fertilization(NF),conventional farmland without fertilization(NO),all with furrow-ridge film mulching cultivation for up to 7 years.the main results are as follows:(1)Alfalfa grassland standing for 30 years had serious soil desiccation,the average water content of 0-500 cm soil profile was only 9.2%,the minimum is as low as 7.6%(the soil permanent wilting point in this region: alfalfa grassland is about 6.5%,the corn field is about 9%).Although the content of soil organic carbon and total nitrogen of alfalfa crops rotation farmland was significantly higher than that of conventional farmland,but the soil total phosphorus,available phosphorus and mineralized nitrogen were significantly lower than those of conventional farmland.Especially the soil available phosphorus and mineralized nitrogen,their content of alfalfa crops rotation farmland was only 39% and 25% of the conventional farmland.(2)In the case without fertilization,alfalfa crop rotation farmland reached yield level of conventional farmland from the third year of testing.Under the fertilization condition,alfalfa crop rotation farmland also reached yield level of conventional farmland from the fourth year of testing.During 7 years of testing,the average yield of conventional farmland plus fertilization was 35% higher than that of conventional farmland without fertilization,increased from 5367 Kg/ha to 7258 Kg/ha,the yield increasing range was 756-3022 Kg/ha.The average yield of fertilization was 32% higher than that of non-fertilization in alfalfa crops rotation farmland,increased from 4605 Kg/ha to 6068 Kg/ha,the yield increasing range was 1117-2222 Kg/ha.fertilization can significantly increase crop yield of alfalfa crops rotation farmland through improving water use efficiency,but there is no difference between total amount of crop water consumption of fertilization and non-fertilization at all.(3)The soil moisture of alfalfa crops rotation farmland recovered gradually with the increase of crop planting years.For the first two years(2010.4-2012.4),recovery occurred mainly in 0-100 cm profile,and soil water storage increased from 106 mm to 170mm;in the second two years(2012.4-2014.4),recovery occurred mainly in 100-200 cm profile,and soil water storage increased from 113 mm to 188mm;in the third two years(2014.4-2016.4),soil moisture restoration mainly occurred in 200-360 cm,and soil water storage by 155 mm increased to 247 mm.(4)The recovery of soil water not only occurs in the growing season with more precipitation,but also in the farmland fallow months with less precipitation,and sometimes the total amount of soil water recovery in the latter was even higher than that of the former.During the first 4 years of rapid moisture recovery of 0-200 cm soil profile(2010 to 2014),the total amount of water recovery in the 4 growing seasons was 16.9mm,while the recovery total amount in the fallow months of 4 years was 80.7mm.(5)The content of total nitrogen in the surface soil of the alfalfa crops rotation farmland declined continuously with the extension of crop planting years,and decreased by 5.6% in 7 years(from 0.9 g/Kg to 0.85 g/Kg,about 0.05g/Kg).However,during the 7 years,total nitrogen content was always significantly higher than conventional farmland,while soil mineralized nitrogen was not higher than conventional farmland for the above reason.fertilization could significantly increase total nitrogen and mineralized nitrogen content in surface soil of alfalfa crops rotation farmland.There was even an upward tendency of the mineralized nitrogen in the surface soil of alfalfa crops rotation farmland plus fertilization,but the content of mineralized nitrogen in the alfalfa crops rotation farmland without fertilization and conventional farmland was all in steady state during test.The content of total nitrogen in 15-60 cm profile of alfalfa crop rotation farmland was significantly higher than that in conventional farmland.However,it would decreased as the crop planting years went by,and would gradually converged to that of conventional farmland.(6)The content of total phosphorus in the surface soil of the alfalfa crops rotation farmland increased gradually during test,but it was still always significantly lower than conventional farmland,Moreover,the content of soil available phosphorus was significantly lower than that of conventional farmland,and showed a downward trend with crop planting time.After 3 testing years,fertilization can significantly increase available phosphorus content of both alfalfa crops rotation farmland and conventional farmland,and also could reduce the rate of reduce of available phosphorus and increase the rate of increase of the carbon phosphorus ratio.(7)The content of soil organic carbon of alfalfa crop rotation farmland increased first and then decreased during test period,after 7 years of testing,it was reduced by 4.5% compared to that in the beginning of the test,about 0.4g/Kg.however,it is still significantly higher than that of conventional farmland on the whole,and in the latter period of the test,fertilization significantly increased soil organic carbon content of alfalfa crop rotation farmland.Consistent with soil total nitrogen,the content of soil organic carbon in 15-60 cm profile of alfalfa crop rotation farmland was significantly higher than that in conventional farmland.However,it would decreas as the crop planting years went by,and would gradually converged to that of conventional farmland.In summary,furrow-ridge film mulching cultivation and appropriate fertilization could significantly promote alfalfa crop rotation farmland rapidly gaining equal yield with conventional farmland,and effectively improve the soil carbon,nitrogen and phosphorus,so as to maintain and improve the quality of soil in alfalfa crop rotation farmland. |
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