The amount, but not the proportion, of N₂ fixation and transfers to neighboring plants varies across grassland soils

Figures & data

Table 1. Soil properties.

Soils	HH	CN	HW
pH	5.27	5.15	4.58
Total C (g C kg^−1 DWE)	51.9	17.7	40.5
Total N (g N kg^−1 DWE)	5.35	1.97	3.99
C:N	9.70	9.01	10.16
NH4^+ (mg N kg^−1 DWE)	47.6	24.5	33.1
NO3^− (mg N kg^−1 DWE)	14.7	15.5	13.0
Exchangeable P (mg P kg^−1 DWE)	8.48	15.45	27.0
Exchangeable K (mg K kg^−1 DWE)	149	113	145
Exchangeable Ca (g Ca kg^−1 DWE)	1.28	0.87	0.36
Exchangeable Mg (mg Mg kg^−1 DWE)	116	69.0	60.0

HH, CN, and HW are Hallsworth, Crediton, and Halstow series, respectively. DWE = dry weight equivalent. Values are mean (n = 2).

Table 2. Nitrogen concentration and δ¹⁵N of the reference grass, and the grass and red clover grown together, after harvesting.

	N concentration (%)			δ^15N (‰)
Soils	R-grass	Grass	Clover	R-grass	Grass	Clover
HH	1.55 ± 0.11 a	1.40 ± 0.11 a	1.89 ± 0.05 b	4.05 ± 0.79 a	2.19 ± 0.42 a	0.56 ± 0.57 a
CN	0.74 ± 0.05 b	0.62 ± 0.07 b	1.79 ± 0.12 b	1.01 ± 0.10 b	0.43 ± 0.31 b	−0.53 ± 0.44 b
HW	0.82 ± 0.04 b	0.65 ± 0.04 b	2.40 ± 0.33 a	0.54 ± 0.07 b	0.03 ± 0.17 b	−0.66 ± 0.22 b

HH, CN and HW are the Hallsworth, Crediton, and Halstow soil series, respectively. R-grass refers to the reference grass which was planted in isolation. Superscript letters indicate significant differences between plants within soils (p < 0.05; ANOVA with Duncan’s test, PASW Statistics 18.0).

Figure 1. Grass and clover aboveground biomass yields for the three soils. HH, CN, and HW are the Hallsworth, Crediton, and Halstow soil series, respectively. Error bars show the standard error of the treatment mean (n = 5) and different letters represent significant differences between values within soils (p < 0.05).

Figure 2. The amount (ANdfa; a) and proportion (PNdfa; b) of N in red clover derived from the atmosphere. HH, CN, and HW are the Hallsworth, Crediton, and Halstow soils, respectively. Error bars show the standard error of the treatment mean (n = 5) and different letters represent significant differences between values (p < 0.05).

Figure 3. The relationship between the total amount of fixed N in clover and the aboveground biomass.

Figure 4. The amount (ANdfc,a) and proportion (PNdfc,b) of N in grass transferred from clover. HH, CN, and HW are Hallsworth, Crediton, and Halstow soil series, respectively. Error bars show the standard error of treatment means (n = 5), different letters represent significant differences between values (p < 0.05).

Figure 5. The relationship between the quantity of N in the grass shoot transferred from the red clover and total grass aboveground biomass.

Figure 6. The δ¹⁵N in soil planted with grass or with grass and red clover together. HH, CN, and HW are Hallsworth, Crediton, and Halstow soil series, respectively. Error bars show the standard error of the treatment mean values (n = 5), and the different letters represent significant differences between values within soil planted grass with or without clover (p < 0.05).