Abstract
Nitrogen (N) pulse is a frequent event in agroecosystems caused by fertilization. Understanding the responses of nitrogen metabolisms in native crops and invasive weeds to N pulses is essential in investigating the invasive mechanism of invasive weeds. A pot experiment was carried out to study the impacts of N pulse and the interspecific competition on nitrogen metabolism of an invasive weed (Amaranthus retroflexus) and a native crop (Glycine max); the plants were applied with an equal amount of N in three N pulse treatments, i.e., sole-summit treatment (SS) with N only applied on the seeding date, double-summit treatment (DS) with twice N applied (the fertilizer was applied on both the seeding date and the flowering date), and no-summit treatment (NS) in which N was applied evenly during the experiment. The results showed that A. retroflexus increased the nitrate reductase (NR) activity more than G. max (except for the roots) in the early growing stage, and increased the glutamine synthetase (GS) and glutamate dehydrogenase (GDH) activities in stem more than G. max in SS and DS treatments during the last two growing stages, however, the advantages were far weaker in the NS treatment. Interspecific competition had negative effects on the nitrogen metabolism of the two species among most of the sample times, and the effects of interspecific competition exerted a tissue-specific influence on nitrogen metabolism in the two species. A. retroflexus switched to reproductive growth earlier in SS treatment than in the DS and NS treatments when it was grown in mixed planting, and its height was the lowest in the NS treatment, so the competitive ability of A. retroflexus was higher in the SS and DS treatments than in the NS treatment, while SS treatment was the common application method of N fertilizer in the G. max farmland in China. Thus, the results of this study suggest that, if the farmer changed the N fertilizer application mode to a constant multiple fertilization mode, the competitive capacity of A. retroflexus will be reduced.
Highlights
The nitrate and ammonium contents in the two species were strongly impacted by N fertilizer application treatments, except for G. max leaves (P < 0.05) (Table 1)
On the third sample time, the two species response to N fertilizer application treatments were idiosyncratic, and the nitrate contents of the two species showed tissue-specific changes in response to N fertilizer application treatments (Figure 2), while the ammonium contents of G. max were maximizing in the double-summit treatment (DS) treatment, the ammonium contents of A. retroflexus were maximizing in the no-summit treatment (NS) treatment, except for the stems in the mixed planting (Figure 3)
On the third sample time, the two species showed a different trend; nitrate reductase (NR) activities of G. max peaked in sole-summit treatment (SS) treatment, whereas NR activities in leaves and stems of A. retroflexus peaked in DS treatment, and NR activities in roots and reproductive organs of A. retroflexus peaked in NS treatment (Figure 4)
Summary
The exotic weed would undergo intense and various pulses caused by the agricultural activities, and the exotic weed could be well-suited to the pulses conditions of agro-ecosystems [10]; exploring the eco-physiological responses of the invasive weed to resource pulses will contribute considerably to our understanding of the invasive mechanisms of the exotic weed. The research on the responses of enzymes activities related to N metabolism to different nitrogen pulses strategies would contribute to our understanding of the exotic weed invasive mechanisms in agro-ecosystem. To investigate the effects of N pulses on the nitrogen metabolism of invasive weed and native crops, an experiment was conducted with Amaranthus retroflexus and Glycine max. 3 o3f 1o6f 16 metabolism enzyme activities in mixed culture than in pure culture in the variable nitrogen supply cocnudlittuiorenst,hwanheinrepausrGe.cmualtxuwreoiunltdhsehvoawriathbeleonpiptroosgiteentsruenpdp.ly conditions, whereas G. max would show the opposite trend
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