Belowground biomass C outweighs soil organic C of perennial energy crops: Insights from a long‐term multispecies trial

Abstract

The cultivation of perennial energy crops (PECs) couples the production of ligno‐cellulosic biomass to the provision of multiple ecosystem services, such as the reduction of greenhouse gas emissions and the mitigation of climate change through carbon (C) sequestration in soil. Though C sequestration in soil by PECs has been widely studied, the contribution of their belowground biomass (BGB) to soil C sequestration and their influence on soil nitrogen (N) storage potential has received very little attention. In this study, C and N stocks in soil and BGB fractions (plant belowground organs and fine roots) were measured for six PECs (Populus spp. ‘Poplar’, Robinia pseudoacacia ‘Black locust’, Salix spp. ‘Willow’, Arundo donax ‘Giant reed’, Miscanthus × giganteus ‘Miscanthus’ and Panicum virgatum ‘Switchgrass’) grown on marginal soil, 11 years after establishment. All PECs had a higher soil organic carbon (SOC) stock and soil total nitrogen (STN) stock than arable land in the top (0–10 cm) soil layer. In this same top layer, woody crops had the highest SOC stock. The increase in SOC under PECs led to increased soil porosity in the top‐soil layer. On average, 43% of the belowground C stock of PECs was allocated in the plant belowground organs (PBO; i.e. in the rhizomes of herbaceous PECs and the stump for woody PECs). Giant reed had the highest C stock in PBO, whereas switchgrass the lowest (22.7 vs. 5.9 Mg C ha−1). On the contrary, switchgrass had the highest C stock in fine roots. Giant reed had the highest belowground C stock (sum of soil and BGB contribution) and black locust the highest belowground N stock. After 11 years of PEC cultivation, 68% of the belowground C stock was allocated in the BGB, and 32% was as SOC.

Short-Term Mineralization of Belowground Biomass of Perennial Biomass Crops after Reversion to Arable Land

“…Perennial biomass crops (PBCs) are low-input crops with the potential to mitigate climate change by sequestering soil organic carbon (SOC) [1,2] and to sustain the provision of multiple ecosystem services [3][4][5]. However, crop yield of PBCs tends to decrease after 15-20 years [6,7], and for their economic sustainability it would be profitable to end their cultivation and convert the soil to the cultivation of annual crops.…”

“…Agronomy 2022, 12, 485 2 of 18 Conserving C and nutrients locked up into soil organic matter (SOM) during the cultivation phase is a priority in perennial cropping systems being reconverted to arable land. PBCs can store huge amounts of C in belowground biomass (BGB) that is incorporated into the soil at the reversion [2,15]. BGB of PBCs is represented by plant belowground organs (PBO), such as rhizomes and/or stumps and fine roots (FR).…”

“…BGB of PBCs is represented by plant belowground organs (PBO), such as rhizomes and/or stumps and fine roots (FR). Martani et al [2] reported, for example, that C sequestered in BGB (18 Mg C ha −1 ) after 11 y in 0-30 cm soil layer by BGB is almost two times higher than the amount of C sequestered in soil. The fate of this C pool after reversion and its impact on SOC-cycling after reversion are still unknown.…”

“…The fate of this C pool after reversion and its impact on SOC-cycling after reversion are still unknown. Concerns exist, that the reversion can lead to a fast decrease in soil quality [16], offsetting at least partially the climate benefits of PBCs [9], but the pulse incorporation of BGB in soil has the potential to save C after the reversion [2]. The effect of BGB incorporation on SOC trajectories largely depends on the removal method and the management of the following land use.…”

Nutrient (N, P, K) use efficiency and nitrogen balance in a bioenergy plantation of Populus deltoides clone Lux in San Piero A Grado (Pisa), Italy

“…Nitrogen balance. As it emerges from the results obtained, and taking into account that the nitrogen of the belowground organs of poplars is estimated at about 200 kg per hectare in an 11-year-old plantation (Martani et al 2021), for both cutting cycles, there is loss of nitrogen in the system. In this sense, it is very important to evaluate the fertilization doses that should be used, since otherwise, we would be decreasing the amount of nitrogen available for the correct growth of the plant.…”