Abstract
Cotton root growth can be affected by different nitrogen fertilizer rates. The objective of the present study was to quantify the effects of nitrogen fertilization rate on cotton root growth and distribution using minirhizotron and soil coring methods. A secondary objective was to evaluate the minirhizotron method as a tool for determining nitrogen application rates using the root distribution as an index. This study was conducted on a Bt cotton cultivar (Jimian 958) under four nitrogen fertilization rates, i.e., 0, 120, 240 and 480 kg ha-1 (control, low, moderate and high levels, respectively), in the Yellow River basin of China from 2013–2015. The sampling process, details of each method as well as the root morphology and root distribution were measured. The operational processes, time and labor needed for the soil core method were all greater than those for the minirhizotron method. The total root length density and the length density in most soil layers, especially in the upper soil layers, first increased but then decreased as nitrogen fertilization increased, and the same trend was observed for both methods. Compared with N0, the total root length density under moderate nitrogen fertilization by the soil coring method increased by more than 94.82%, in 2014 and 61.11% in 2015; while by the minirhizotron method the corresponding values were 28.24% in 2014 and 57.47%, in 2015. Most roots were distributed in the shallow soil layers (0–60 cm) in each method. However, the root distribution with the soil coring method (>73.11%) was greater than that with the minirhizotron method (>47.07%). The correlations between the root morphology indexes of shallow soil depth measured using the two methods were generally significant, with correlative coefficients greater than 0.334. We concluded that the minirhizotron method could be used for cotton root analysis and most cotton roots distributed in upper soil layers (0-60cm). In addition, a moderate nitrogen rate (240 kg ha-1) could increase root growth, especially in the shallow soil layers. The differences observed with the minirhizotron method were clearer than those observed with the soil coring method.
Highlights
Roots play an important role in plant growth, as they are in direct contact with the soil and are the main organs that absorb water and nutrition [1]
Fertilizer affects root distribution, cotton root length density, root volume density in the 0–40 cm soil layer is decreased by nitrogen application [13]
The time needed for each sample ranged from 10 to 80 minutes; In minirhizotron method, all the roots are marked on a computer with a mouse
Summary
Roots play an important role in plant growth, as they are in direct contact with the soil and are the main organs that absorb water and nutrition [1]. Root biomass are affected by nitrogen fertilization [7]. Adequate nitrogen can improve the root length density and root activity in the 40–120 cm soil layer under mulch drip irrigation [10, 11]. Nitrogen fertilizer reduces root length, root surface area, volume, and diameter under drip irrigation with saline water [12]. Fertilizer affects root distribution, cotton root length density, root volume density in the 0–40 cm soil layer is decreased by nitrogen application [13]. Nitrogen application can increase cotton root nitrate reductase activity and protective enzyme [14,15,16]. Research on the effects of nitrogen fertilization on the roots morphology and distribution of cotton under field conditions is of great importance
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