Stability of hydrolytic enzyme activity and microbial phosphorus during storage of tropical rain forest soils

Abstract

Storage can markedly influence microbial and biochemical properties in soils, yet recommendations for sample storage are based on studies of temperate soils that regularly experience extended periods of cold temperature. We assessed the influence of storage conditions on microbial phosphorus and the activity of four hydrolytic enzymes (phosphomonoesterase, phosphodiesterase, [beta]-glucosidase, and N-acetyl-[beta]-d-glucosaminidase) in three lowland tropical forest soils from the Republic of Panama that experience a constant warm temperature. The soils spanned a strong rainfall gradient and contained contrasting physical and chemical properties (pH 3.6-5.9; total carbon 26-50 g C kg-1; clay 33-62%; total phosphorus 0.30-0.60 g P kg-1). Storage treatments were: (i) room temperature (22 °C in the dark), (ii) refrigerated (4 °C in the dark), (iii) air-dried (10 d, 22 °C), and (iv) frozen (-35 °C). There were significant changes in enzyme activities and microbial phosphorus during refrigerated and room temperature storage, although changes were relatively small during the first two weeks. An initial marked decline in enzyme activities for one soil analyzed within 2 h of sampling was attributed to a flush of activity caused by sampling and soil preparation (sieving, etc.). For longer-term storage (>2 weeks), ambient laboratory temperature appeared preferable to freezing and cold storage, because one month of storage caused a marked decline in enzyme activities and microbial phosphorus in one soil. Freezing preserved the activities of some enzymes in some soils at rates comparable to cold or room temperature storage, but caused a marked decline in microbial phosphorus for two soils. Air-drying caused a marked decline in microbial phosphorus and the activity of all enzymes. We therefore conclude that enzyme assays and microbial phosphorus should be determined in tropical forest soils after no more than two weeks storage in the dark at ambient laboratory temperature.