半干旱黄土丘陵区苜蓿人工草地土壤磷素有效性及对生产力的响应

研究了半干旱黄土丘陵区以垄沟集雨技术建成的紫花苜蓿（Medicago sativa L．）人工草地对土壤磷索的影响。土壤全磷在处理之间和采样期之间均无显著差异。但在各处理土壤中速效磷在试验期间的减少量和苜蓿的干草产量成正比，垄上覆盖薄膜的2个垄沟处理（M30-垄和沟宽度均为30em，M60-垄和沟宽度均为60cm）比平作（CK）的干草产量分别显著提高了10．7％和40．3％，两个未覆盖的垄沟处理（B30-垄和沟宽度均为30cm，B60-垄和沟宽度均为60em）干草产量分别比平作对照下降了14．2％和28．3％。相应地，3a试验期间速效磷的减少量为M60（55．5％）〉M30（51．5％）〉CK（34．6％）〉B30（23．4％）〉B60（17．5％）。并且在3年试验后，所有处理的土壤有机碳和速效磷的比值（C／P比）均比播种前有显著增加，其中M30和M60的C／P比分别达到1165．1和1326．1，显著高于其他处理。试验还发现，在干旱年份土壤有机碳和有效磷显著正相关，而在湿润年份二者为显著负相关。要进一步提高苜蓿人工草地产草量或延长草地高产年限，必须寻找增加土壤有效磷的途径或方法。

Availability and contributions of soil phosphorus to forage production of seeded alfalfa in semiarid Loess Plateau

In the semiarid loess hilly and gully region of Northwest China, the availabilities of water and phosphorus (P) are the main limitations on seeded alfalfa (Medicago sativa L.) production. The water-harvesting technique can improve the soil water condition. However, the contribution of availability of soil phosphorus to the production of seeded alfalfa established by this technique is unknown. In this research, we studied the availability of soil phosphorus in response to the production of seeded alfalfa established by the ridges and furrows water-harvesting technique. The following five treatments were set up in this study: 1) conventional cultivation in a flat plot without mulch (CK), 2) plastic mulched ridge with 30 cm width of ridge and furrow (M30), 3) plastic mulched ridge with 60 cm width of ridge and furrow (M60), 4) bare ridge with 30 cm width of ridge and furrow (B30), and 5) bare ridge with 60 cm width of ridge and furrow (B60). The forage yield, evapotranspiration (ET), soil organic carbon (SOC), total phosphorus, available phosphorus, and the ratio of soil organic C to available P (C/P) were measured in the experiment. Results showed that the total soil P was not significant different among treatments and sampling dates. However, the positive correlation relationship occurred between the decrease in soil available P and total forage yield of alfalfa after 3-year alfalfa stand. The forage yield of alfalfa in the two ridge-mulched treatments (M30 and M60) were higher than that of conventional flat cultivation (CK) by 10.7% and 40.3%, respectively, while the yield of two bare treatments (B30 and B60) were lower than that of CK by 14.2% and 28.3%, respectively. After a 3-year alfalfa stand, the trend of the decreases in soil available P was in the order of M60 (55.5%)> M30 (51.5%)> CK (34.6%)> B30 (23.4%)> B60 (17.5%). Moreover, after 3-year alfalfa growth, the C/P ratio increased significantly in all of the treatments comparing to that before sowing. The C/P ratio of M30 (1165.1) and M60 (1326.1) were significantly higher than that of the other treatments. More interesting finding was that the increase in SOC in dry years promoted the accumulation of soil available P. However, in wet years, the increase of SOC limited the accumulation of soil available P. Therefore, to further increase the forage yield of alfalfa, the effective technique must be developed to increase the soil available P.