Why copper and zinc are ineffective in reducing soil urease activity in New Zealand dairy-grazed pasture soils

Micronutrients copper (Cu) and zinc (Zn) have the potential to inhibit soil urease activity (UA) and reduce ammonia (N[H.sub.3]) emissions over long duration (8-12 weeks) but have not been tested for reducing N[H.sub.3] losses from cattle urine deposited in dairy-grazed pasture soils. The objective of this study was to assess the effectiveness and longevity of Cu and Zn in reducing soil UA, for the use of these metals to reduce N[H.sub.3] emissions from deposited urine by grazing cattle. A series of experiments were conducted to (i) assess the relationship between inherent Cu and Zn status and soil UA of New Zealand dairy-grazed pasture soils, (ii) determine the impact of Cu and Zn addition to pasture soils on soil UA and (iii) investigate how soil organic carbon (C) and other C-related textural and mineralogical properties such as clay content and cation exchange capacity influence the effectiveness of added Cu and Zn in reducing urea hydrolysis. The results showed significant positive correlations of soil total C and total nitrogen (N) with soil UA. However, there were no significant negative correlations of soil UA with inherent Cu and Zn levels. Similarly, addition of Cu and Zn to soil did not significantly reduce soil UA. However, when Cu was added to two different soil supernatants there was a significant reduction in hydrolysis of urea applied at 120 and 600 mg urea-N [kg.sup.-1] soil. Additions of Zn achieved negligible or small reductions in urea hydrolysis after 120 and 600 mg urea-N [kg.sup.-1] soil applications to soil supernatants. This result suggests that Cu can inhibit soil UA and urea hydrolysis in soil supernatants with potentially low C, clay and cation exchangeable base contents. However, the interaction of bioavailable Cu with labile soil organic C and clay particles leads to its inactivation, resulting in ineffectiveness in organic C-rich pasture soils. Although most of the added Zn did not complex and remained bioavailable, the observed levels of bioavailable Zn had limited effect on soil UA. Additional keywords: ammonia emissions, clay content and cation exchange capacity, metal bioavailability, nitrogen loss, nitrogen use efficiency, soil organic carbon, urease inhibition.
Introduction Intensification of dairy farming in New Zealand (NZ) over the last three decades has resulted in a substantial increase in the national dairy herd size from 2.3 million cows [...]