Phosphorus partitioning and phosphatase activity in semi-arid region soils under increasing crop growth intensity

Reduction in the use of fallow in traditional wheat-fallow cropping systems, along with other management changes such as reduced tillage, raise questions about nutrient management, particularly phosphorus (P) management, in such soils. Phosphorus is focused on because of its low soil mobility and its common application in wheat cropping systems. Consequently, we initiated a study with the following objectives: (i) Measure changes in soil available P status during the fallow year under two different cropping intensities; (ii) Determine differences in P status with soil depth and topographic position; (iii) Relate changes in P fractions (organic, inorganic) to P availability; and (iv) Relate P availability (as measured by Bray and Kurtz [B&K] extraction) to soil phosphatase activity. We sampled soils by depth, topographic position, and season during the fallow year in farmers’ fields used for wheat-fallow (w-f) or wheat-proso millet-fallow (w-m-f) cropping systems located near Sidney, Nebraska. Soils were analyzed for B&K-P, total P (TP), inorganic P (IP), organic P (OP), organic matter (OM), and phosphatase activity. Soils under w-m-f were up to 5 mg P kg soil higher in B&K-P than those under w-f, but differences were not significant. Soils at footslope positions were significantly higher in OP, IP, TP, OM, and phosphatase activity, probably because of a combination of deposition of eroded soil and higher average soil moisture content. Differences in P-related properties across seasons were inconsistent. B&K-P was strongly correlated to phosphatase activity, OM, IP, and TP (all P < 0.0001). The general absence of differences in any measured quantity between cropping systems suggests that cropping intensity can be increased in semiarid climates such as the study site, at least over the short-term, without concerns of inducing P deficiencies. Phosphatase activity, although related to P availability, does not offer greater benefits in predicting soil P supply than established soil tests.