Effect of tractor traffic and N fertilization on the root morphology of grass/red clover mixture

Tractor traffic and related soil compaction may create unfavourable growing conditions for the development of roots and above-ground biomass production. It can be expected that higher N fertilization and higher below-ground biomass production can remediate soil physically degraded by wheel compaction. The main objective of this study was to investigate the interaction between tractor traffic and N fertilization of a clover/grass mixture during the period from 2010 to 2012. The experiment was established in a split-plot design with fertilization as a main plot and tractor passes as a subplot. The N fertilizer treatments used were: untreated control (N0), 80 kg N ha(-1) (N80) and 160 kg N ha(-1) (N160). Four compaction treatments were applied using the following numbers of tractor passes: untreated control (P0), two passes (P2), four passes (P4) and six passes (P6). Root samples were collected in the autumn of 2010, 2011 and 2012 and their morphometric parameters were calculated after measurement of the root length, using image analysis: root length density (RLD), mean root diameter (MRD), specific root length (SRL) and root dry matter (RDM). Heavy tractor traffic significantly affected the physical parameters of the soil. The highest value of bulk density was recorded in the 10-20 cm soil layer with the P6 treatment (1.582 g cm(-3)), being approximately 10% higher compared with the P0 treatment at the same depth. The tractor wheeling increased penetration resistance in the 0-20 cm soil layer. The maximum value of penetration resistance (2.98 MPa) was recorded for the P6 treatment, in the 5-10 cm soil layer. The differences between root parameters caused by fertilization and compaction treatments were observed only in the 5-15 cm soil layer. The roots in the upper, 0-5 cm, soil layer were not affected by soil compaction or nitrogen fertilization. The tractor passes reduced biomass and length of the roots. In the 5-10 cm soil layer the highest value of the RDM was noticed with the P0 treatment (0.00241 g cm(-3)), whilst it decreased to 0.00144 g cm(-3) for the P6 treatment. In the 10-15 cm the maximum value for the RDM was also recorded in the P0 whereas the minimum was in the P4. The higher N rates play unfavourable role in root development what reflected in their morphometric parameters. The use of nitrogen reduced the RDM. The main difference was noticed between the N160 treatment (0.00230 g cm(-3)) and two others: namely, N0 and N80, totalling 0.00298 and 0.000287 g cm(-3) respectively. Similar reductions were also observed for the RLD. The results in root morphology did not indicate significant interaction between tractor passes and the N fertilization level. According to the regression model, the optimum condition for root growth could be meet when nitrogen fertilization is at the level of approximately 50 kg N ha(-1). At this N rate red clover/grass mixture produces the highest below ground biomass what can play favourable role in remediation of compacted soil. (C) 2013 Elsevier B.V. All rights reserved.