Comparison of Ingrowth-Core and Sequential Soil Core Methods on the Belowground Net Primary Production Estimation in Ryegrass-Clover Swards.

The temperate grassland is a global atmospheric carbon sink where the belowground net primary productivity (BNPP, g m^-2) creates the largest organic carbon input to grassland soils. To elucidate the belowground carbon sequestration in grassland especially in response to the long-term management effect, improving estimation accuracy of BNPP is of key importance where its coefficient of variation (CV, %) is, however, reviewed by others as 50-75% of measured means currently. In this work, the ingrowth-core method was performed in ryegrass-clover swards for BNPP investigation with: short-term (2-4.5 weeks), medium-term (5-9 weeks) and long-term (27 weeks) ingrowth periods replicated in 2010 and 2011; contemporarily the “maximum-minimum” and “all positive increment” calculations based on soil core method were also adopted. By methodological comparison, the medium-term ingrowth-core provided the most reliable BNPP estimate (495 g m^-2; CV: 34%). The ingrowth period is the supreme factor on reliability. Combining our results with other grassland ingrowth-core studies, a 4-6 weeks’ ingrowth period is recommended to minimize both installation disturbance and root detachment. Due to lack of seasonality in belowground biomass, the two soil core calculations only produced results with obvious underestimation and higher uncertainty thus invalid. It is concluded that the ingrowth-core method fits well for belowground carbon sequestration investigation in productive grassland sites, especially appreciable for its lower systematic variation thus able to resolve treatments’ effect on the input of belowground carbon, facilitating long-term carbon budget simulation.