Root Productivity and Turnover in Native Prairie

Quantitative measurements of root systems on a seasonal basis for a 34—inch profile in Central Missouri Prairie ranged from 1, 449 g/m 2 prior to resumption of growth in the spring to 1, 901 g/m 2 at the end of the growing season. Underground parts in the upper 2 in. of the profile varied from 48% of the total quantity in April to 60% in July. Over 80% of the root mass occurred in the A₁ horizon, or the top 10 in. of the profile. Annual increment in the A₁ horizon or surface 10 in. was 429 g/m 2. In the A₂ horizon, 10—18 in. , the measurement was 40 g/m 2 and in the B 2 horizon, 18—30 in. , 41 g/m 2. These values were approximately 25% of the maximum dry matter measurement for each profile level, suggesting a turnover for the root system as a whole every 4 years. Variations in turnover for different parts of the root system were indicated, however, as shown by an annual loss of 22. 8% for rhizomes, and 40. 8% for roots in the 0— to 2—in. zone. Decomposition constants showed a marked decrease with depth, ranging from 4. 3% in the A₁ horizon to 0. 8% in the B 2 horizon. Estimated time required to reach 99% organic matter equilibrium under present conditions of production and breakdown was 110, 420, and 590 years for the A₁, A₂, and B 2 horizons respectively. Increasing time values would be expected with increasing depth as the reduction in annual increment was proportionately greater at lower levels in the profile than that for the total root—humus product.