The "Birch Effect" - Understanding Soil Organic Matter Mineralization in Dry Regions.

H. F. Birch was a soil scientist working in the British colonial service at the East African Agriculture and Forestry Research Organization in Kikuyu, Kenya. From 1955 to 1962 he published many papers reporting the results of studies incubating soils in a respirometer that automatically replenished the oxygen as microbes converted it to CO₂. Lacking today’s sophisticated instruments for in situ measurements, he cleverly devised experiments that produced essentially circumstantial evidence for the mechanisms involved in the burst of respiration and nitrification that occurs when dried soils are rewetted. This phenomenon has become known as the "Birch Effect." His hand drawn CO₂ and nitrate release curves with repeated drying and wetting cycles are classic illustrations of the Birch Effect. He interpreted his data broadly: “…results indicate that part, at least, of the humus fraction under moist conditions is protected from, but not inert to, microbial attack.” “Further, since decomposition generally parallels ammonia accumulation there should be a quick release, through mineralization, of nitrate and other nutrients. Such behaviour has an obvious bearing on (a) the seasonal pattern of humus decomposition, (b) the nitrate flush which…occurs at the start of the rains and is related to the degree of soil drying…” The principals of the Birch Effect are still being applied in ecological studies ranging from large scale carbon cycling projects using Eddie-Variance towers to measure CO₂ fluxes from semi-arid ecosystems to soil test kits that attempt to monitor soil health by the size of the CO₂ burst on rewetting a dried soil sample.