Rhizosphere calcareous soil P-extraction at the expense of organic carbon from root-exuded organic acids induced by phosphorus deficiency in several plant species

Figures & data

Table 1 Characteristics of rhizosphere soil on which different plant species grew in Karst soil. Note: values are means ± standard error (SE) (n = 5)

	Broussonetia papyrifera L. Vent (B. papyrifera)	Morus alba L. (M. alba)	Orychophragmus violaceus L. Schulz (O. violaceus)	Brassica napus L. (B. napus)
Original pH (1:1 H2O)	7.81 ± 0.09	7.55 ± 0.06	7.75 ± 0.13	7.59 ± 0.09
Available phosphorus (mg kg^−1)	4.90 ± 0.09	3.43 ± 0.11	4.21 ± 0.03	3.54 ± 0.04
Organic carbon (g kg^−1)	35.9 ± 0.11	47.4 ± 0.23	43.7 ± 0.28	46.5 ± 0.19
CaCO3 (g kg^−1)	47.6 ± 0.16	46.9 ± 0.14	48.3 ± 0.13	49.4 ± 0.20
Total nitrogen (g kg^−1)	2.88 ± 0.06	2.79 ± 0.09	3.14 ± 0.11	2.47 ± 0.07

Table 2 Root dry weight and dry weight increase of four plant species as affected by phosphorus (P) treatments. Note: values are means ± standard error (SE) (n = 3). The different small letters indicate the significant difference in four plant species under two P treatments at p < 0.05

Plant species	P treatment	Root dry weight (g per plant)		Increase in root dry weight during 50 days’ treatment (g per plant)	The percentage inhibition of root dry weight (%)
		0 days	50 days
Broussonetia papyrifera L. Vent (B. papyrifera)	P-normal	0.732 ± 0.094	0.892 ± 0.036	0.156 ± 0.023	−21.86 ± 2.83 a
	P-deficient	0.807 ± 0.023	0.725 ± 0.010	−0.082 ± 0.016	8.92 ± 1.06 d
Morus alba L. (M. alba)	P-normal	0.653 ± 0.047	0.706 ± 0.033	0.050 ± 0.009	−8.12 ± 1.09 c
	P-deficient	0.685 ± 0.051	0.599 ± 0.029	−0.086 ± 0.002	12.55 ± 1.66 d
Orychophragmus violaceus L. Schulz (O. violaceus)	P-normal	0.764 ± 0.044	0.873 ± 0.012	0.109 ± 0.011	−14.27 ± 2.08 b
	P-deficient	0.778 ± 0.038	0.694 ± 0.033	−0.084 ± 0.006	10.80 ± 1.77 d
Brassica napus L. (B. napus)	P-normal	0.598 ± 0.063	0.681 ± 0.041	0.083 ± 0.004	−13.88 ± 1.91 b
	P-deficient	0.675 ± 0.067	0.487 ± 0.024	−0.188 ± 0.013	27.85 ± 3.49 e

Table 3 Relationships between rhizosphere soil phosphorus (P)-extraction and exogenous organic acid concentration (≤ 10 mM) for different plant species and organic acids. Note: Y is the amount of P extraction from rhizosphere soil (mg kg⁻¹ DW soil); X is the organic acid concentration (mM). ** indicates significant difference at p < 0.001; * indicates significant difference at p < 0.005. Per carbon P-extraction expresses the amount of rhizosphere soil P extraction by a mole weight of carbon from CA, or OA, or MA

Plant species	Organic acid	Regression equation^a	Determination coefficient (R^2)	Per carbon P-extraction
Broussonetia papyrifera L. Vent (B. papyrifera)	Citric acid	Y = 5.397X + 2.040	0.998**	0.900
	Oxalic acid	Y = 4.545X + 2.166	0.996**	2.273
	Malic acid	Y = 2.970X + 1.850	0.986**	0.743
Morus alba L. (M. alba)	Citric acid	Y = 2.510X + 1.544	0.997**	0.418
	Oxalic acid	Y = 2.171X + 1.341	0.996**	1.086
	Malic acid	Y = 1.222X + 1.649	0.992**	0.306
Orychophragmus violaceus L. Schulz (O. violaceus)	Citric acid	Y = 2.178X + 3.827	0.972**	0.363
	Oxalic acid	Y = 1.941X + 3.540	0.976**	0.971
	Malic acid	Y = 1.526X + 3.356	0.999**	0.382
Brassica napus L. (B. napus)	Citric acid	Y = 2.078X + 2.463	0.991**	0.346
	Oxalic acid	Y = 0.743X + 2.501	0.975**	0.372
	Malic acid	Y = 0.457X + 2.229	0.942*	0.114

Figure 1 The sum of root-exuded oxalic, malic, and citric acids in (A) woody Moraceae plants [B. papyrifera (Bp) and M. alba (Ma)] and (B) herbaceous cruciferous plants [O. violaceus (Ov) and B. napus (Bn)] from the root during 6 h in 10, 20, 30, 40 and 50 treatment days.

Note: blocks with bars indicate mean ± standard error (SE). The mean ± SE with the same letter are not statistically different at p = 0.05. Small letters refer to differences between phosphorus (P) treatments in the same plant species. Capital letters refer to differences between plant species under the same organic acid. DW, dry weight.

Figure 2 The increment proportion of phosphorus extraction (IP_PEX) by the root-exuded organic acids of (A) woody Moraceae plants [B. papyrifera (Bp) and M. alba (Ma)] and (B) herbaceous cruciferous plants [O. violaceus (Ov) and B. napus (Bn)] in phosphorus (P)-deficient compared with P-normal plants during different treatment times.

Note: circles and triangles with bars indicate mean ± standard error (SE). Means ± SE with the same letter are not statistically different at p = 0.05. Small Latin letters refer to differences between two plant species under the same treatment time. Small Greek letters refer to differences between treatment times under the same plant species.

Figure 3 Loss of organic carbon (LOC) caused by root-exuded organic acids of (A) woody Moraceae plants [B. papyrifera (Bp) and M. alba (Ma)] and (B) herbaceous cruciferous plants [O. violaceus (Ov) and B. napus (Bn)] in phosphorus (P)-deficient compared with P-normal plants during different treatment times.

Note: circles and triangles with bars indicate mean ± standard error (SE). Means ± SE with the same letter are not statistically different at p = 0.05. Small Latin letters refer to differences between two plant species under the same treatment time. Small Greek letters refer to differences between treatment times in the same plant species.

Figure 4 Phosphorus (P)-extraction cost caused by root-exuded organic acids in (A) woody Moraceae plants [B. papyrifera (Bp) and M. alba (Ma)] and (B) herbaceous cruciferous plants [O. violaceus (Ov) and B. napus (Bn)] in P-deficient compared with P-normal during different treatment times.

Note: circles and triangles with bars indicate mean ± standard error (SE). Means ± SE with the same letter are not statistically different at p = 0.05. Small Latin letters refer to differences between two plant species under the same treatment time. Small Greek letters refer to differences between treatment times in the same plant species.