Microbial Biomass in Soils as Influenced by Different Nutrients

貧栄養土壌のマサ土における土壌微生物バイオマスの形成に必要な無機養分を比較検討するために、土壌培養実験を行った。処理は、N、P、K、S、Ca、Mg全てを添加した完全区と完全区から各要素を除いた-N、-P、-K、-S、-Ca、-Mg区の7処理区を設け、マサ土を最大容水量の60%に水分調整した後、各処理区とも無機養分と1Kg土壌当たり1000MgCをグルコースで同時に添加し、25℃で10日間培養し、5日目と10日目の微生物バイオマス量と土壌呼吸量を測定した。微生物バイオマスは、養分添加によって5日目まで増加しその後低下したが、5日目、10日目のいずれにおいても、同様な処理間差異が認められた。すなわち微生物バイオマス量は、完全区で最も高く、Mg、K、Ca、S、P、N欠如区の順に低下し、各処理区の完全区に対する相対バイオマス量は、Mg、K、Ca、S、P、N欠如区で各々91-97、80-91、74-81、67-72、62-66、52-55%とN、P、S、Ca欠如区で著しく低かった。また微生物バイオマス活性はバイオマス量の多い区ほど低い傾向が認められた。これらの結果から、マサ土における土壌微生物バイオマスの形成には、CやNばかりでなく、P、S、Caなどの供給が必要であることが明らかとなった。

An incubation experiment was conducted to evaluate the effects of several mineral nutrients application on the soil microbial biomass formation in a granitic regosol of Japan which have very low chemical fertility. Several nutrients (N, P, K, Ca, Mg and S) application treatment was assigned as a control plot and each nutrient reduction treatment was considered as (-N), (-P), (-K), (-Ca), (-Mg) and (-S) plot. These mineral nutrients of each plot were applied as the nutrient solution with 1000 mg C kg-1 soil (glucose) to the regosol and the amount of microbial biomass was compared with the control. The amount of microbial biomass C markedly increased with each nutrient application, and the maximum values of microbial biomass of each plot were significantly different among the nutrients applied. While, the specific respiration of microbial biomass was decreased with increasing microbial biomass C. Relative values of microbial biomass C of each plot over the control was quite similar at 5 days and 10 days after nutrients application, it was the lowest at -N plot (52-55%) followed by -P (62-65%), -S (67-72%), -Ca (74-81%), -K (80-91%) and -Mg (91-96%) plot, respectively. From these results, it was concluded that not only C and N but also P, S and Ca application was essentially required for microbial biomass formation in the granitic regosol of Japan.