3.6.Six types of carbon sources
There were significant differences in the use ratios of the six types of carbon source between mulberry and alfalfa treatments (Fig. 4). The relative use rates of carbohydrates, polymers, and miscellaneous substances were higher in mulberry than in alfalfa treatments. However, the relative use rates of carboxylic acids and amino acids were higher in alfalfa treatments. The use ratios of carbohydrates (30.79-37.51%), polymers (21.14-23.96%), and miscellaneous substrates (15.32-18.98%) in mulberry treatments were higher than in the corresponding alfalfa treatments, namely carbohydrates (28.06-31.27%), polymers (13.94-14.89%), and miscellaneous substrates (6.20-7.30%), respectively. The use ratios of carboxylic acids (19.29-20.24%), amino acids (23.24-26.38%), and amines/amides (4.17-5.32%) in alfalfa treatments were higher than those in mulberry treatments, namely acids (11.43-14.97%), amino acids (7.09-8.37%), and amines/amides (3.11-5.77%). These results indicate that mulberry and alfalfa have strategically and complementarily used the six carbon sources. Nitrogen application and intercropping decreased the use ratio of carbohydrates in mulberry treatments, but increased it in alfalfa treatments. The use ratios of amino acids in alfalfa intercropped with mulberry were lower than those in monoculture alfalfa systems, irrespective of nitrogen application. However, nitrogen application decreased the use ratios of amino acids in mulberry, irrespective of the cropping system. The use ratios of polymers in the intercropping of mulberry and alfalfa were higher than those in the monoculture; for mulberry, this was irrespective of nitrogen application. The use ratios of carboxylic acids were higher in mulberry with nitrogen application, without any significant differences between AE, ANE, and AN0. The utilization ratios of amine/amides in alfalfa treatments with nitrogen were lower than those without nitrogen, irrespective of the cropping system.
Among all treatments, the relative use rates of 31 carbon sources differed significantly (Fig. 5). Specifically, the number of carbon sources of the relative use rates exceed 4% was up to 17 in ME, 15 in M0, 13 in MNE, 9 in MN0, 12 in AE, 7 in A0, 21 in ANE, and 19 in AN0; 2-hydroxy benzoic acid and α-D-lactose were scarcely used in all treatments. In addition, γ-hydroxybutyric acid, L-threonine, and α-ketobutyric acid were scarcely use in mulberry treatments (ME, M0, MNE, MN0) and in alfalfa intercropped with mulberry treatments (AE and A0). The relative use rates of D-malic acid, phenylethyl-amine, and glycyl-L-glutamic acid in all treatments did not exceed 4%. On the contrary, the relative use rates of L-asparagine, D-glucosamine acid, L-serine, D-mannito, D-cellobiose, N-acetyl-D-glucose, and mehtyl-D-glucoside (except for A0) exceeded 4% in all treatments. In addition, the relative use rate of D-glucosamine acid in alfalfa exceeded 4%, while those of Tween 40 and Methyl-D-glucoside exceeded 4% in all treatments except for A0. Nitrogen application and intercropping increased the relative use rates of L-arginine and α-cyclodextrin, which exceeded 4% in treatments of mulberry (ME, M0, MNE), while the relative use rate of L-arginine in treatments of alfalfa exceeded 4% except for A0. For α-cyclodextrin and itaconic acid, the rates exceeded 4% in the treatments AE and ANE, while for putrescine and D-galactonic-acid-lactone, they exceeded 4% in treatments of mulberry with nitrogen application (MNE and ME). The relative use rates of D-glucosaminic acid, glucose-l-phosphate, and Tween 80 exceeded 4% in treatments of intercropped mulberry (M0 and ME), while for pyruvic acid methyl ester and D-xylose, the rates exceeded 4% in intercropped alfalfa with nitrogen application. The relative use rates of 4-hydroxy benzoic acid, putrescine, D-galactonic acid lactone, glucose-1-phosphate, Tween 80, and glycogen exceeded 4% in AN0 and ANE, but were below 4% in intercropping systems with mulberry (AE and A0).