To date, little attention has been paid to the effects of leaf source reduction on photosynthetic matter production, root function and post-silking N uptake characteristics at different planting densities. In a 2-year field experiment, Xianyu 335, a widely released hybrid in China, was planted at 60 000 plants ha^–1 (conventional planting density, CD) and 90 000 plants ha^–1 (high planting density, HD), respectively. Until all the filaments protruded from the ear, at which point the plants were subjected to the removal of 1/2 (T1), 1/3 (T2) and 1/4 (T3) each leaf length per plant, no leaf removal served as the control (CK). We evaluated the leaf source reduction on canopy photosynthetic matter production and N accumulation of different planting densities. Under CD, decreasing leaf source markedly decreased photosynthetic rate (P_n), effective quantum yield of photosystem II (ΦPSII) and the maximal efficiency of photosystem II photochemistry (F_v/F_m) at grain filling stage, reduced post-silking dry matter accumulation, harvest index (HI), and the yield. Compared with the CK, the 2-year average yields of T1, T2 and T3 treatments decreased by 35.4, 23.8 and 8.3%, respectively. Meanwhile, decreasing leaf source reduced the root bleeding sap intensity, the content of soluble sugar in the bleeding sap, post-silking N uptake, and N accumulation in grain. The grain N accumulation in T1, T2 and T3 decreased by 26.7, 16.5 and 12.8% compared with CK, respectively. Under HD, compared to other treatments, excising T3 markedly improved the leaf P_n, ΦPSII and F_v/F_m at late-grain filling stage, increased the post-silking dry matter accumulation, HI and the grain yield. The yield of T3 was 9.2, 35.7 and 20.1% higher than that of CK, T1 and T2 on average, respectively. The T3 treatment also increased the root bleeding sap intensity, the content of soluble sugar in the bleeding sap and post-silking N uptake and N accumulation in grain. Compared with CK, T1 and T2 treatments, the grain N accumulation in T3 increased by 13.1, 40.9 and 25.2% on average, respectively. In addition, under the same source reduction treatment, the maize yield of HD was significantly higher than that of CD. Therefore, planting density should be increased in maize production for higher grain yield. Under HD, moderate decreasing leaf source improved photosynthetic performance and increased the post-silking dry matter accumulation and HI, and thus the grain yield. In addition, the improvement of photosynthetic performance improved the root function and promoted post-silking N uptake, which led to the increase of N accumulation in grain.

The adsorption of Cu(II) from aqueous solution onto humic acid (HA) which was isolated from cattle manure (CHA), peat (PHA), and leaf litter (LHA) as a function of contact time, pH, ion strength, and initial concentration was studied using the batch method. X-ray absorption spectroscopy (XAS) was used to examine the coordination environment of the Cu(II) adsorbed by HA at a molecular level. Moreover, the chemical compositions of the isolated HA were characterized by elemental analysis and solid-state 13C nuclear magnetic resonance spectroscopy (NMR). The kinetic data showed that the adsorption equilibrium can be achieved within 8 h. The adsorption kinetics followed the pseudo-second-order equation. The adsorption isotherms could be well fitted by the Langmuir model, and the maximum adsorption capacities of Cu(II) on CHA, PHA, and LHA were 229.4, 210.4, and 197.7 mg g–1, respectively. The adsorption of Cu(II) on HA increased with the increase in pH from 2 to 7, and maintained a high level at pH>7. The adsorption of Cu(II) was also strongly influenced by the low ionic strength of 0.01 to 0.2 mol L–1 NaNO3, but was weakly influenced by high ionic strength of 0.4 to 1 mol L–1 NaNO3. The Cu(II) adsorption on HA may be mainly attributed to ion exchange and surface complexation. XAS results revealed that the binding site and oxidation state of Cu adsorbed on HA surface did not change at the initial Cu(II) concentrations of 15 to 40 mg L–1. For all the Cu(II) adsorption samples, each Cu atom was surrounded by 4 O/N atoms at a bond distance of 1.95 Å in the first coordination shell. The presence of the higher Cu coordination shells proved that Cu(II) was adsorbed via an inner-sphere covalent bond onto the HA surface. Among the three HA samples, the adsorption capacity and affinity of CHA for Cu(II) was the greatest, followed by that of PHA and LHA. All the three HA samples exhibited similar types of elemental and functional groups, but different contents of elemental and functional groups. CHA contained larger proportions of methoxyl C, phenolic C and carbonyl C, and smaller proportions of alkyl C and carbohydrate C than PHA and LHA. The structural differences of the three HA samples are responsible for their distinct adsorption capacity and affinity toward Cu(II). These results are important to achieve better understanding of the behavior of Cu(II) in soil and water bodies in the presence of organic materials.