Below-ground plant biomass and nitrogen uptake of perennial forage grasses and annual crops fertilized with pig manures

The aim of this study was to determine the importance of below-ground (root) plant biomass and nitrogen (N) uptake with perennial cropping system (PCS) as a mechanism of reducing nitrate leaching. The experimental design was a split plot with main plot being cropping system [annual cropping system (ACS) versus PCS] and subplots being manure treatment [N-based liquid pig manure (LPM), N-based solid pig manure (SPM), and a control with no manure addition (CON)]. The ACS was seeded to barley (Hurdeum vulgare L.) and canola (Brassica napus L.) in the spring of 2014 and 2015, respectively, whereas the PCS was seeded to a mixture of timothy (Phleum protense L.) and orchard (Dactylis glomerata L.) grass in the fall of 2013. LPM and SPM were applied to meet N requirement of each crop in spring of each year. In 2014, above-ground and root biomass were collected at harvest and in 2015, above-ground and root biomass were collected at mid-season. The PCS had significantly greater root biomass and N uptake than ACS in both years. Root biomass in PCS ranged from 5.3-9.7 Mg ha−1 compared to a range of 1.1-1.5 Mg ha−1 in ACS. Consequently, root N uptake in PCS ranged from 43 to 118 kg N ha−1 in both years while that in ACS was 9-20 kg N ha−1. In 2014, PCS had significantly greater above-ground N than ACS, whereas in 2015, PCS had significantly greater above-ground N than ACS in the LPM only. This study shows that the differences in nitrate leaching between the two cropping systems as previously determined from field core lysimeters at the same plots was mainly due to differences in root N uptake (34-98 kg N ha−1) between the cropping systems rather than above-ground N uptake. It is important that studies comparing N losses by ACS and PCS should consider the role of plant root biomass in N recovery.