Effects of reclamation duration on soil nutrients and enzyme activities in the plough layer and subsoil of riparian floodplains

Croplands reclaimed from floodplain wetlands play an important role in food production. As reclamation con-tinues, various agronomic measures affect soil nutrient contents and metabolism. In the lower reaches of the Yellow River, we selected croplands (wheat and maize double-cropped system) reclaimed for 13, 24, and 33 years from natural wetlands (as the control), with similar starting soil characteristics and management practices. We determined total nitrogen (TN) and phosphorus (TP) contents and enzyme activities (including invertase, dehydrogenase, beta-glucosidase, urease, protease, acid phosphatase, and alkaline phosphatase) in 0-100 cm soil profile. As results, TN and TP contents increased following reclamation, particularly in the plough layer (0-20 cm) with a clear increase during the chronosequence. The subsoil (20-100 cm) accounted for 55.3-63.9% and 66.6-68.0% of the N and P stocks in croplands, respectively. The TN content was positively correlated with the silt proportion in the 100-cm profile, whereas TP content was positively correlated with the clay proportion in the plough layer, and negatively with pH in the subsoil. Nevertheless, the C: N and C: P ratios in the plough layer were higher in the croplands than in the natural wetlands, and the N: P ratio increased from 0.4 to 2.6 following reclamation. Activities of most enzymes in the plough layer increased along with the chronosequence, and the subsoil generally had higher enzyme activity after 33 years of reclamation than in the natural wetlands. Redundancy analysis revealed that TN content and N: P ratio had strong explanatory power for the enzyme activities in the plough and subsoil layers, respectively. These findings suggest reclamation increases soil nutrient contents and enzyme activities in the floodplains, and the subsoil plays an essential role. However, these croplands remain N deficient. Thus, applying N fertilizer, as well as planting deep-rooted and N2-fixing crops, should be considered for agricultural management.